Iahub is so happy about inventing bubble sort graphs that he's staying all day long at the office and writing permutations. Iahubina is angry that she is no more important for Iahub. When Iahub goes away, Iahubina comes to his office and sabotage his research work.

The girl finds an important permutation for the research. The permutation contains n distinct integers a1, a2, ..., an (1 ≤ ai ≤ n). She replaces some of permutation elements with -1 value as a revenge. 

When Iahub finds out his important permutation is broken, he tries to recover it. The only thing he remembers about the permutation is it didn't have any fixed point. A fixed point for a permutation is an element ak which has value equal to k (ak = k). Your job is to proof to Iahub that trying to recover it is not a good idea. Output the number of permutations which could be originally Iahub's important permutation, modulo 1000000007 (109 + 7).

Input

The first line contains integer n (2 ≤ n ≤ 2000). On the second line, there are n integers, representing Iahub's important permutation after Iahubina replaces some values with -1. 

It's guaranteed that there are no fixed points in the given permutation. Also, the given sequence contains at least two numbers -1 and each positive number occurs in the sequence at most once. It's guaranteed that there is at least one suitable permutation.

Output

Output a single integer, the number of ways Iahub could recover his permutation, modulo 1000000007 (109 + 7).

Examples

Input

5
-1 -1 4 3 -1


Output

2

Note

For the first test example there are two permutations with no fixed points are [2, 5, 4, 3, 1] and [5, 1, 4, 3, 2]. Any other permutation would have at least one fixed point. 
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.IOException;
import java.io.UncheckedIOException;
import java.io.Closeable;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.math.BigInteger;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) throws Exception {
        Thread thread = new Thread(null, new TaskAdapter(), "", 1 << 29);
        thread.start();
        thread.join();
    }

    static class TaskAdapter implements Runnable {
        @Override
        public void run() {
            InputStream inputStream = System.in;
            OutputStream outputStream = System.out;
            FastInput in = new FastInput(inputStream);
            FastOutput out = new FastOutput(outputStream);
            CIahubAndPermutations solver = new CIahubAndPermutations();
            solver.solve(1, in, out);
            out.close();
        }
    }

    static class CIahubAndPermutations {
        int mod = (int) 1e9 + 7;
        Factorial fact = new Factorial((int) 1e4, mod);
        Combination comb = new Combination(fact);

        public void solve(int testNumber, FastInput in, FastOutput out) {
            int n = in.ri();
            int[] P = new int[n];
            boolean[] occur = new boolean[n];
            for (int i = 0; i < n; i++) {
                P[i] = in.ri() - 1;
                if (P[i] >= 0) {
                    occur[P[i]] = true;
                }
            }
            int a = 0;
            for (int x : P) {
                if (x >= 0) {
                    a++;
                }
            }
            int b = n - a;
            int b1 = 0;
            for (int i = 0; i < n; i++) {
                if (P[i] < 0 && !occur[i]) {
                    b1++;
                }
            }
            long ans = 0;
            for (int x = 0; x <= b1; x++) {
                long contrib = 1;
                contrib = contrib * comb.combination(b1, x) % mod
                        * comb.combination(n - a - x, b - x) % mod
                        * fact.fact(b - x) % mod
                        * fact.fact(n - a - b) % mod;
                if (x % 2 == 1) {
                    contrib = -contrib;
                }
                ans += contrib;
            }
            ans = DigitUtils.mod(ans, mod);
            ans = ans * fact.fact(n - a - b) % mod;
            out.println(ans);
        }

    }

    static class FastInput {
        private final InputStream is;
        private byte[] buf = new byte[1 << 13];
        private int bufLen;
        private int bufOffset;
        private int next;

        public FastInput(InputStream is) {
            this.is = is;
        }

        private int read() {
            while (bufLen == bufOffset) {
                bufOffset = 0;
                try {
                    bufLen = is.read(buf);
                } catch (IOException e) {
                    bufLen = -1;
                }
                if (bufLen == -1) {
                    return -1;
                }
            }
            return buf[bufOffset++];
        }

        public void skipBlank() {
            while (next >= 0 && next <= 32) {
                next = read();
            }
        }

        public int ri() {
            return readInt();
        }

        public int readInt() {
            int sign = 1;

            skipBlank();
            if (next == '+' || next == '-') {
                sign = next == '+' ? 1 : -1;
                next = read();
            }

            int val = 0;
            if (sign == 1) {
                while (next >= '0' && next <= '9') {
                    val = val * 10 + next - '0';
                    next = read();
                }
            } else {
                while (next >= '0' && next <= '9') {
                    val = val * 10 - next + '0';
                    next = read();
                }
            }

            return val;
        }

    }

    static class FastOutput implements AutoCloseable, Closeable, Appendable {
        private static final int THRESHOLD = 32 << 10;
        private final Writer os;
        private StringBuilder cache = new StringBuilder(THRESHOLD * 2);

        public FastOutput append(CharSequence csq) {
            cache.append(csq);
            return this;
        }

        public FastOutput append(CharSequence csq, int start, int end) {
            cache.append(csq, start, end);
            return this;
        }

        private void afterWrite() {
            if (cache.length() < THRESHOLD) {
                return;
            }
            flush();
        }

        public FastOutput(Writer os) {
            this.os = os;
        }

        public FastOutput(OutputStream os) {
            this(new OutputStreamWriter(os));
        }

        public FastOutput append(char c) {
            cache.append(c);
            afterWrite();
            return this;
        }

        public FastOutput append(long c) {
            cache.append(c);
            afterWrite();
            return this;
        }

        public FastOutput println(long c) {
            return append(c).println();
        }

        public FastOutput println() {
            return append('\n');
        }

        public FastOutput flush() {
            try {
//            boolean success = false;
//            if (stringBuilderValueField != null) {
//                try {
//                    char[] value = (char[]) stringBuilderValueField.get(cache);
//                    os.write(value, 0, cache.length());
//                    success = true;
//                } catch (Exception e) {
//                }
//            }
//            if (!success) {
                os.append(cache);
//            }
                os.flush();
                cache.setLength(0);
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
            return this;
        }

        public void close() {
            flush();
            try {
                os.close();
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
        }

        public String toString() {
            return cache.toString();
        }

    }

    static class Combination implements IntCombination {
        final Factorial factorial;
        int modVal;

        public Combination(Factorial factorial) {
            this.factorial = factorial;
            this.modVal = factorial.getMod();
        }

        public Combination(int limit, int mod) {
            this(new Factorial(limit, mod));
        }

        public int combination(int m, int n) {
            if (n > m || n < 0) {
                return 0;
            }
            return (int) ((long) factorial.fact(m) * factorial.invFact(n) % modVal * factorial.invFact(m - n) % modVal);
        }

    }

    static interface IntCombination {
    }

    static class Factorial {
        int[] fact;
        int[] inv;
        int mod;

        public int getMod() {
            return mod;
        }

        public Factorial(int[] fact, int[] inv, int mod) {
            this.mod = mod;
            this.fact = fact;
            this.inv = inv;
            fact[0] = inv[0] = 1;
            int n = Math.min(fact.length, mod);
            for (int i = 1; i < n; i++) {
                fact[i] = i;
                fact[i] = (int) ((long) fact[i] * fact[i - 1] % mod);
            }
            inv[n - 1] = BigInteger.valueOf(fact[n - 1]).modInverse(BigInteger.valueOf(mod)).intValue();
            for (int i = n - 2; i >= 1; i--) {
                inv[i] = (int) ((long) inv[i + 1] * (i + 1) % mod);
            }
        }

        public Factorial(int limit, int mod) {
            this(new int[Math.min(limit + 1, mod)], new int[Math.min(limit + 1, mod)], mod);
        }

        public int fact(int n) {
            if (n >= mod) {
                return 0;
            }
            return fact[n];
        }

        public int invFact(int n) {
            if (n >= mod) {
                throw new IllegalArgumentException();
            }
            return inv[n];
        }

    }

    static class DigitUtils {
        private DigitUtils() {
        }

        public static int mod(long x, int mod) {
            if (x < -mod || x >= mod) {
                x %= mod;
            }
            if (x < 0) {
                x += mod;
            }
            return (int) x;
        }

    }
}

