The 9-th grade student Gabriel noticed a caterpillar on a tree when walking around in a forest after the classes. The caterpillar was on the height h1 cm from the ground. On the height h2 cm (h2 > h1) on the same tree hung an apple and the caterpillar was crawling to the apple.

Gabriel is interested when the caterpillar gets the apple. He noted that the caterpillar goes up by a cm per hour by day and slips down by b cm per hour by night.

In how many days Gabriel should return to the forest to see the caterpillar get the apple. You can consider that the day starts at 10 am and finishes at 10 pm. Gabriel's classes finish at 2 pm. You can consider that Gabriel noticed the caterpillar just after the classes at 2 pm.

Note that the forest is magic so the caterpillar can slip down under the ground and then lift to the apple.

Input

The first line contains two integers h1, h2 (1 ≤ h1 < h2 ≤ 105) — the heights of the position of the caterpillar and the apple in centimeters.

The second line contains two integers a, b (1 ≤ a, b ≤ 105) — the distance the caterpillar goes up by day and slips down by night, in centimeters per hour.

Output

Print the only integer k — the number of days Gabriel should wait to return to the forest and see the caterpillar getting the apple.

If the caterpillar can't get the apple print the only integer  - 1.

Examples

Input

10 30
2 1


Output

1


Input

10 13
1 1


Output

0


Input

10 19
1 2


Output

-1


Input

1 50
5 4


Output

1

Note

In the first example at 10 pm of the first day the caterpillar gets the height 26. At 10 am of the next day it slips down to the height 14. And finally at 6 pm of the same day the caterpillar gets the apple.

Note that in the last example the caterpillar was slipping down under the ground and getting the apple on the next day.

import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;
import java.awt.Point;


public class Try
{
    static long mod=1000000007;
    static BigInteger bigInteger = new BigInteger("1000000007");
    static int n = (int)1e6;
    static boolean[] prime;
    static ArrayList<Integer> as;
    static void sieve() {
        Arrays.fill(prime	, true);
        prime[0] = prime[1] = false;
        for(int i = 2 ; i * i <= n ; ++i) {
            if(prime[i]) {
                for(int k = i * i; k< n ; k+=i) {
                    prime[k] = false;
                }
            }
        }

    }
    static PrintWriter w = new PrintWriter(System.out);
    static char [][]sol;
    public static void main(String[] args)
    {

        InputReader sc = new InputReader(System.in);
        //PrintWriter w = new PrintWriter(System.out);
    
       /* 
        prime  = new boolean[n + 1];
		sieve();
		prime[1] = false;
		*/
		/*
		as = new ArrayList<>();
		for(int i=2;i<=1000000;i++)
		{
		    if(prime[i])
		    as.add(i);
		}
		*/

        // MergeSort ob = new MergeSort();
        // ob.sort(arr, 0, arr.length-1);

        // Student []st = new Student[x];
        // st[i] = new Student(i,d[i]);
        //Arrays.sort(st,(p,q)->p.diff-q.diff);
      int x = sc.ni();
      int y = sc.ni();
      int a = sc.ni();
      int b = sc.ni();
      int fa = (a*8) - (b*12);
      x += (8*a);
      if(b >= a)
      {
          x -= (8*a);
          if(x+(8*a) <y)
          {
              w.println("-1");
              
          }
          else
          {
              w.println("0");
          }
              
      }
      else
      {
         int c1 = 0;
         while(x < y)
         {
             x -= (12*b);
             x += (12*a);
             c1++;
         }
         w.println(c1);
      }
        
        
        

      
              
              
         

        w.close();



    }

    static class Student
    {

        int first;
        int sec;
        Student(int first,int sec)
        {
            this.first = first;
            this.sec = sec;


        }

    }

    public static long modMultiply(long one, long two) {

        return (one % mod * two % mod) % mod;
    }
    static long fx(int m)
    {
        long re = 0;
        for(int i=1;i<=m;i++)
        {
            re += (long) (i / gcd(i,m));
        }
        return re;
    }
    static int gcd(int a, int b)
    {
        if (a == 0)
            return b;

        return gcd(b%a, a);
    }



    static long phi(long nx)
    {
        // Initialize result as n
        double result = nx;


        // Consider all prime factors of n and for 
        // every prime factor p, multiply result
        // with (1 - 1/p)
        for (int p = 0; as.get(p) * as.get(p) <= nx; ++p)
        {
            // Check if p is a prime factor.
            if (nx % as.get(p) == 0)
            {
                // If yes, then update n and result
                while (nx % as.get(p) == 0)
                    nx /= as.get(p);
                result *= (1.0 - (1.0 / (double) as.get(p)));
            }
        }

        // If n has a prime factor greater than sqrt(n)
        // (There can be at-most one such prime factor)
        if (nx > 1)
            result *= (1.0 - (1.0 / (double) nx));

        return (long)result;


        //return(phi((long)result,k-1));

    }

    public static void primeFactors(int n,int x)
    {
        as = new ArrayList<>();

        // Print the number of 2s that divide n
        while (n%2==0)
        {


            as.add(2);
            //System.out.print(2 + " ");
            n /= 2;

        }

        // n must be odd at this point.  So we can
        // skip one element (Note i = i +2)
        for (int i = 3; i <= Math.sqrt(n); i+= 2)
        {
            // While i divides n, print i and divide n
            while (n%i == 0)
            {

                // System.out.print(i + " ");

                as.add(i);
                n /= i;


            }

        }

        // This condition is to handle the case whien
        // n is a prime number greater than 2
        if (n >= 2)
        {

            as.add(n);
        }

    }
    static int digitsum(int x)

    {

        int sum = 0;
        while(x > 0)
        {
            int temp = x % 10;
            sum += temp;
            x /= 10;
        }
        return sum;

    }
    static int countDivisors(int n)
    {
        int cnt = 0;
        for (int i = 1; i*i <=n; i++)
        {
            if (n % i == 0 && i<=1000000)
            {
                // If divisors are equal,
                // count only one
                if (n / i == i)
                    cnt++;

                else // Otherwise count both
                    cnt = cnt + 2;
            }
        }
        return cnt;
    }

    static boolean isprime(int n)
    {

        if(n == 2)
            return true;
        if(n == 3)
            return true;
        if(n % 2 == 0)
            return false;
        if(n % 3 == 0)
            return false;

        int i = 5;
        int w = 2;

        while(i * i <= n)
        {
            if(n % i == 0)
                return false;


            i += w;
            w = 6 - w;
        }

        return true;
    }

    static long log2(long value) {
        return Long.SIZE-Long.numberOfLeadingZeros(value);
    }






    static int binarysearch(int []arr,int p,int n)
    {
        //ArrayList<Integer> as = new ArrayList<>();
        //as.addAll(0,at);
        //Collections.sort(as);
        boolean re = false;
        int st = 0;
        int end = n-1;



        while(st < end)
        {
            int mid = (st + end)/2;

            if(p > arr[mid])
            {
                st = mid+1;

            }
            else if(p <= arr[mid])
            {
                end = mid-1;

            }




        }
        return end;

    }



    /* Java program for Merge Sort */
    static class MergeSort
    {
        // Merges two subarrays of arr[].
        // First subarray is arr[l..m]
        // Second subarray is arr[m+1..r]
        void merge(int arr[], int l, int m, int r)
        {
            // Find sizes of two subarrays to be merged
            int n1 = m - l + 1;
            int n2 = r - m;

            /* Create temp arrays */
            int L[] = new int [n1];
            int R[] = new int [n2];

            /*Copy data to temp arrays*/
            for (int i=0; i<n1; ++i)
                L[i] = arr[l + i];
            for (int j=0; j<n2; ++j)
                R[j] = arr[m + 1+ j];


            /* Merge the temp arrays */

            // Initial indexes of first and second subarrays
            int i = 0, j = 0;

            // Initial index of merged subarry array
            int k = l;
            while (i < n1 && j < n2)
            {
                if (L[i] <= R[j])
                {
                    arr[k] = L[i];
                    i++;
                }
                else
                {
                    arr[k] = R[j];
                    j++;
                }
                k++;
            }

            /* Copy remaining elements of L[] if any */
            while (i < n1)
            {
                arr[k] = L[i];
                i++;
                k++;
            }

            /* Copy remaining elements of R[] if any */
            while (j < n2)
            {
                arr[k] = R[j];
                j++;
                k++;
            }
        }

        // Main function that sorts arr[l..r] using
        // merge()
        void sort(int arr[], int l, int r)
        {
            if (l < r)
            {
                // Find the middle point
                int m = (l+r)/2;

                // Sort first and second halves
                sort(arr, l, m);
                sort(arr , m+1, r);

                // Merge the sorted halves
                merge(arr, l, m, r);
            }
        }

        /* A utility function to print array of size n */



    }





    public static int ip(String s){
        return Integer.parseInt(s);
    }
    public static String ips(int s){
        return Integer.toString(s);
    }


    static class InputReader {

        private final InputStream stream;
        private final byte[] buf = new byte[8192];
        private int curChar, snumChars;

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

        public int read() {
            if (snumChars == -1)
                throw new InputMismatchException();
            if (curChar >= snumChars) {
                curChar = 0;
                try {
                    snumChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (snumChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }

        public int ni() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public long nl() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            long res = 0;
            do {
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public int[] nia(int n) {
            int a[] = new int[n];
            for (int i = 0; i < n; i++) {
                a[i] = ni();
            }
            return a;
        }

        public String rs() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = read();
            } while (!isSpaceChar(c));
            return res.toString();
        }

        public String nextLine() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = read();
            } while (!isEndOfLine(c));
            return res.toString();
        }

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

        private boolean isEndOfLine(int c) {
            return c == '\n' || c == '\r' || c == -1;
        }

    }






}