Note that the only difference between String Transformation 1 and String Transformation 2 is in the move Koa does. In this version the letter y Koa selects must be strictly greater alphabetically than x (read statement for better understanding). You can make hacks in these problems independently.

Koa the Koala has two strings A and B of the same length n (|A|=|B|=n) consisting of the first 20 lowercase English alphabet letters (ie. from a to t).

In one move Koa:

  1. selects some subset of positions p_1, p_2, …, p_k (k ≥ 1; 1 ≤ p_i ≤ n; p_i ≠ p_j if i ≠ j) of A such that A_{p_1} = A_{p_2} = … = A_{p_k} = x (ie. all letters on this positions are equal to some letter x).

  2. selects a letter y (from the first 20 lowercase letters in English alphabet) such that y>x (ie. letter y is strictly greater alphabetically than x).

  3. sets each letter in positions p_1, p_2, …, p_k to letter y. More formally: for each i (1 ≤ i ≤ k) Koa sets A_{p_i} = y.

Note that you can only modify letters in string A.




Koa wants to know the smallest number of moves she has to do to make strings equal to each other (A = B) or to determine that there is no way to make them equal. Help her!

Input

Each test contains multiple test cases. The first line contains t (1 ≤ t ≤ 10) — the number of test cases. Description of the test cases follows.

The first line of each test case contains one integer n (1 ≤ n ≤ 10^5) — the length of strings A and B.

The second line of each test case contains string A (|A|=n).

The third line of each test case contains string B (|B|=n).

Both strings consists of the first 20 lowercase English alphabet letters (ie. from a to t).

It is guaranteed that the sum of n over all test cases does not exceed 10^5.

Output

For each test case:

Print on a single line the smallest number of moves she has to do to make strings equal to each other (A = B) or -1 if there is no way to make them equal.

Example

Input


5
3
aab
bcc
4
cabc
abcb
3
abc
tsr
4
aabd
cccd
5
abcbd
bcdda


Output


2
-1
3
2
-1

Note

  * In the 1-st test case Koa: 
    1. selects positions 1 and 2 and sets A_1 = A_2 =  b (\color{red}{aa}b → \color{blue}{bb}b). 
    2. selects positions 2 and 3 and sets A_2 = A_3 =  c (b\color{red}{bb} → b\color{blue}{cc}). 

  * In the 2-nd test case Koa has no way to make string A equal B.

  * In the 3-rd test case Koa: 
    1. selects position 1 and sets A_1 =  t (\color{red}{a}bc → \color{blue}{t}bc). 
    2. selects position 2 and sets A_2 =  s (t\color{red}{b}c → t\color{blue}{s}c). 
    3. selects position 3 and sets A_3 =  r (ts\color{red}{c} → ts\color{blue}{r}). 

import java.io.IOException;
import java.io.InputStream;
import java.util.*;

public class TestClass {

    static final class InputReader {
        private final InputStream stream;
        private final byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        private int read() throws IOException {
            if (curChar >= numChars) {
                curChar = 0;
                numChars = stream.read(buf);
                if (numChars <= 0) {
                    return -1;
                }
            }
            return buf[curChar++];
        }

        public final int readInt() throws IOException {
            return (int) readLong();
        }

        public final long readLong() throws IOException {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
                if (c == -1) throw new IOException();
            }
            boolean negative = false;
            if (c == '-') {
                negative = true;
                c = read();
            }
            long res = 0;
            do {
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return negative ? -res : res;
        }

        public final int[] readIntArray(int size) throws IOException {
            int[] array = new int[size];
            for (int i = 0; i < size; i++) {
                array[i] = readInt();
            }
            return array;
        }

        public final long[] readLongArray(int size) throws IOException {
            long[] array = new long[size];
            for (int i = 0; i < size; i++) {
                array[i] = readLong();
            }
            return array;
        }

        public final String readString() throws IOException {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            StringBuilder res = new StringBuilder();
            do {
                res.append((char) c);
                c = read();
            } while (!isSpaceChar(c));
            return res.toString();
        }

        private boolean isSpaceChar(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }
    }

    static long mulmod(long a, long b,
                       long mod) {
        long res = 0; // Initialize result
        a = a % mod;
        while (b > 0) {
            // If b is odd, add 'a' to result
            if (b % 2 == 1) {
                res = (res + a) % mod;
            }

            // Multiply 'a' with 2
            a = (a * 2) % mod;

            // Divide b by 2
            b /= 2;
        }

        // Return result
        return res % mod;
    }

    static long pow(long a, long b, long MOD) {
        long x = 1, y = a;
        while (b > 0) {
            if (b % 2 == 1) {
                x = (x * y);
                if (x > MOD) x %= MOD;
            }
            y = (y * y);
            if (y > MOD) y %= MOD;
            b /= 2;
        }
        return x;
    }

    static long[] f = new long[100011];

    static long InverseEuler(long n, long MOD) {
        return pow(n, MOD - 2, MOD);
    }

    static long C(int n, int r, long MOD) {

        return (f[n] * ((InverseEuler(f[r], MOD) * InverseEuler(f[n - r], MOD)) % MOD)) % MOD;
    }


    static int[] h = {0, 0, -1, 1};
    static int[] v = {1, -1, 0, 0};


    static class Pair2 {
        public long cost;
        int node;

        public Pair2(long cos, int node) {
            this.cost = cos;
            this.node = node;
        }
    }



    static long l[][] = new long[100011][6];
    static long compute_hash(String s) {
        int p = 31;
        int m = 1000000007;
        long hash_value = 0;
        long p_pow = 1;
        for (int i = 0; i < s.length(); ++i) {
            char c = s.charAt(i);
            hash_value = (hash_value + (c - 'a' + 1) * p_pow) % m;
            p_pow = (p_pow * p) % m;
        }
        return hash_value;
    }

    public static class SegmentTree {
        int[][] tree;
        long[] sumtree;
        long[] count;
        int n;

        public SegmentTree(int[] nodes) {
            tree = new int[nodes.length * 4][31];
            sumtree = new long[nodes.length * 4];
            count = new long[nodes.length * 4];
            n = nodes.length;
            build(0, n - 1, 0, nodes);
        }

        private void build(int l, int r, int pos, int[] nodes) {
            if (l == r) {
                sumtree[pos] = nodes[l];
                boolean neg = false;
                if (nodes[l] < 0) {
                    neg = true;
                }
                count[pos] = 1;
                if (neg) {
                    count[pos] = -1;
                }
                for (int i = 0; i < 31; ++i) {
                    tree[pos][i] = (neg ? -1 : 1) * (((nodes[l] < 0 ? -1 * nodes[l] : nodes[l]) & (1 << i)) != 0 ? 1 : 0);
                }
                return;
            }
            int mid = (l + r) / 2;
            build(l, mid, 2 * pos + 1, nodes);
            build(mid + 1, r, 2 * pos + 2, nodes);
            sumtree[pos] = sumtree[2 * pos + 1] + sumtree[2 * pos + 2];
            count[pos] = count[2 * pos + 1] + count[2 * pos + 2];

            for (int i = 0; i < 31; ++i) {
                tree[pos][i] = tree[2 * pos + 1][i] + tree[2 * pos + 2][i];
            }
        }

        public long getSum(int l, int r) {
            return getSumUtil(0, n-1, 0, l, r);
        }

        private long getSumUtil(int l, int r, int pos, int ql, int qr) {
            if (l > qr || r < ql) return 0L;
            if (l >= ql && r <= qr) {
                return sumtree[pos];
            }
            int mid = (l + r) >> 1;
            return getSumUtil(l, mid, 2 * pos + 1, ql, qr) + getSumUtil(mid + 1, r, 2 * pos + 2, ql , qr);
        }

        public long getCount(int l, int r) {
            return getCountUtil(0, n-1, 0, l, r);
        }

        private long getCountUtil(int l, int r, int pos, int ql, int qr) {
            if (l > qr || r < ql) return 0L;
            if (l >= ql && r <= qr) {
                return count[pos];
            }
            int mid = (l + r) >> 1;
            return getCountUtil(l, mid, 2 * pos + 1, ql, qr) + getCountUtil(mid + 1, r, 2 * pos + 2, ql , qr);
        }

        public int[] getBitCount(int l, int r) {
            return getBitCountUtil(0, n-1, 0, l, r);
        }

        private int[] getBitCountUtil(int l, int r, int pos, int ql, int qr) {
            if (l > qr || r < ql) return null;
            if (l >= ql && r <= qr) {
                int[] returs = new int[30];
                for (int i = 0; i < 30; ++i) {
                    returs[i] =  tree[pos][i];
                }
                return returs;
            }
            int mid = (l + r) >> 1;

            int[] left = getBitCountUtil(l, mid, 2 * pos + 1, ql, qr);
            int[] right = getBitCountUtil(mid + 1, r, 2 * pos + 2, ql, qr);
            if (left == null) return right;
            if (right == null) return left;
            // int[] returs = new int[30];
            for (int i = 0; i < 30; ++i) {
                left[i] +=  right[i];
            }
            return left;
        }


    }

    static int counter = 0;
    static int[] In;
    static int[] Out;
    private static int[] flatten;
    private static int[][] lca;
    static long answer = 0;

    static int VISITED = 1;
    static int VISITING = 2;

    public static void main(String[] args) throws Exception {
        //https://i...content-available-to-author-only...e.com/ebRGa6
        InputReader in = new InputReader(System.in);

//        l[0][0] = 1;
//        for (int i = 1; i < 100008; i++) {
//            // Set every nCr = 1 where r = 0
//            l[i][0] = 1;
//            for (int j = 1; j < 5; j++) {
//
//                // Value for the current cell of Pascal's triangle
//                l[i][j] = (l[i - 1][j - 1] + l[i - 1][j]) % 1000000007;
//            }
//        }



        int t = in.readInt();

        while (t-- > 0) {
            int n = in.readInt();

            String a = in.readString();
            String b = in.readString();
            List<Set<Integer>> occ = new ArrayList<>();
            for (int i = 0; i < 20; ++i) {
                occ.add(new HashSet<>());
            }
            boolean hasAns = true;
            for (int i = 0; i < a.length(); ++i) {
                if (b.charAt(i) < a.charAt(i)) {
                    hasAns = false;
                    break;
                }
                occ.get(a.charAt(i) - 'a').add(b.charAt(i) - 'a');
            }

            if (hasAns) {
                int answer = 0;
                for (int i = 0; i < 20; ++i) {
                    Set<Integer> thisChar = occ.get(i);
                    int j = i+1;
                    for (; j < 20; ++j) {
                        if (thisChar.contains(j)) break;
                    }
                    if (j != 20) {
                        occ.get(j).addAll(thisChar);
                        ++answer;
                    }
                }
                System.out.println(answer);
            } else {
                System.out.println(-1);
            }
        }




    }

    private static long solve(int pos, int curK, int k, int[] arr, int m, long[][] dp) {
        System.out.println(pos);
        if (pos == arr.length) {
            if (curK == k) return 0;
            return -1000000000000L;
        }

        if (curK > k) return -1000000000000L;

        if (dp[pos][curK] != -1) return dp[pos][curK];

        long with = -1000000000000L;
        if (curK < k)
            with = solve(pos + 1, curK + 1, k, arr, m, dp) + arr[pos] * (((curK + 1) % m));
        long without = solve(pos + 1, curK, k, arr, m, dp);

        return dp[pos][curK] = Math.max(with, without);
    }

    static int countFreq(String pattern, String text) {
        int m = pattern.length();
        int n = text.length();
        int res = 0;

        for (int i = 0; i <= n - m; i++) {
            int j;
            for (j = 0; j < m; j++) {
                if (text.charAt(i + j) != pattern.charAt(j)) {
                    break;
                }
            }
            if (j == m) {
                res++;
                j = 0;
            }
        }
        return res;
    }


    private static void dfs(List<List<Integer>> g, int node, int last, int[] val, int[] traversal, int depth) {
        In[node] = counter;
        flatten[counter++] = val[node];

        traversal[depth] = node;

        for (int i = 0; depth - (1 << i) >= 0; ++i) {
            lca[node][i] = traversal[depth - (1 << i)];
        }

        for (int i = 0; i < g.get(node).size(); ++i) {
            int ntb = g.get(node).get(i);
            if (ntb != last) {
                dfs(g, ntb, node, val, traversal, depth + 1);
            }
        }

        Out[node] = counter;
        flatten[counter++] = -1 * val[node];
    }


    private static void preprocess(int pos, int[][] pre, List<List<Integer>> tree, int[] traverse, int depth, int last, int[] tin, int[] tout) {
        tin[pos] = counter++;
        traverse[depth] = pos;

        for (int i = 0; depth - (1 << i) >= 0; ++i) {
            pre[pos][i] = traverse[depth - (1 << i)];
        }

        for (int i = 0; i < tree.get(pos).size(); ++i) {
            if (tree.get(pos).get(i) != last)
                preprocess(tree.get(pos).get(i), pre, tree, traverse, depth + 1, pos, tin, tout);
        }
        tout[pos] = counter++;
    }

    static int gcd(int a, int b) {

        while (b != 0) {
            int t = a;
            a = b;
            b = t % b;
        }
        return a;
    }


    static boolean submit = true;

    static void debug(String s) {
        if (!submit)
            System.out.println(s);
    }

    static void debug(int s) {
        if (!submit)
            System.out.println(s);
    }


    class Solution {
        public class Pair {
            public int start, end, c;

            public Pair(int start, int end, int c) {
                this.start = start;
                this.end = end;
                this.c = c;
            }
        }

        public List<String> maxNumOfSubstrings(String s) {
            int n = s.length();
            List<String> answer = new ArrayList<>();
            Map<Integer, Integer> f = new HashMap<>();
            Map<Integer, Integer> l = new HashMap<>();


            for (int i = 0; i < n; ++i) {
                if (!f.containsKey(s.charAt(i) - 'a')) {
                    f.put(s.charAt(i) - 'a', i);
                }
            }

            for (int i = n - 1; i >= 0; --i) {
                if (!l.containsKey(s.charAt(i) - 'a')) {
                    l.put(s.charAt(i) - 'a', i);
                }
            }

            List<Pair> p = new ArrayList<>();
            for (int i = 0; i < 26; ++i) {
                int start = f.getOrDefault(i, -1);
                int end = l.getOrDefault(i, -1);
                if (start != -1) {
                    for (int j = start; j < end; ++j) {
                        end = Math.max(end, l.get(s.charAt(j) - 'a'));
                        if (start > f.get(s.charAt(j) - 'a')) {
                            start = Math.min(start, f.get(s.charAt(j) - 'a'));
                            j = start;
                        }

                    }
                    p.add(new Pair(start, end, i));
                }
            }

            p.sort(Comparator.comparingInt(a -> a.end));
            int lastEnd = 0;
            for (Pair pair : p) {
                if (pair.start >= lastEnd) {
                    answer.add(s.substring(pair.start, pair.end + 1));
                    lastEnd = pair.end;
                }
            }

            return answer;
        }

    }


}

































