#include <iostream>

using namespace std;

typedef long long ll;
template <typename T = long long>
T floor_sum(T n, T m, T a, T b) {
    T ans = 0;
    if (m < 0) {
        m = -m; a = -a; b = -b;
    }
    if (a < 0) {
        T a2 = (a % m + m) % m;
        ans -= (n - 1) * n * ((a2 - a) / m) / 2;
        a = a2;
    }
    if (b < 0) {
        T b2 = (b % m + m) % m;
        ans -= n * ((b2 - b) / m);
        b = b2;
    }
    if (a >= m) {
        ans += (n - 1) * n * (a / m) / 2;
        a %= m;
    }
    if (b >= m) {
        ans += n * (b / m);
        b %= m;
    }
    T y_max = a * (n - 1) + b;
    if (y_max < m) return ans;
    return ans + floor_sum((a * n + b) / m, a, m, (a * n + b) % m);
}

template <typename T = long long>
T ceil_sum(T n,T m,T a,T b){
    return floor_sum<T>(n,m,a,b + m - 1);
}

// int128入出力用ライブラリ
// copied from: https://kenkoooo.hatenablog.com/entry/2016/11/30/163533

#include <bits/stdc++.h>
using namespace std;

std::ostream &operator<<(std::ostream &dest, __int128_t value) {
  std::ostream::sentry s(dest);
  if (s) {
    __uint128_t tmp = value < 0 ? -value : value;
    char buffer[128];
    char *d = std::end(buffer);
    do {
      --d;
      *d = "0123456789"[tmp % 10];
      tmp /= 10;
    } while (tmp != 0);
    if (value < 0) {
      --d;
      *d = '-';
    }
    int len = std::end(buffer) - d;
    if (dest.rdbuf()->sputn(d, len) != len) {
      dest.setstate(std::ios_base::badbit);
    }
  }
  return dest;
}

__int128 parse(string &s) {
  __int128 ret = 0;
  for (int i = 0; i < s.length(); i++)
    if ('0' <= s[i] && s[i] <= '9')
      ret = 10 * ret + s[i] - '0';
  return ret;
}

typedef __int128 i128;
int main(){
    ll a,b,c,d; cin >> a >> b >> c >> d;
    if(a*d==b*c){
        cout << -1 << "\n";
        return 0;
    }
    if(a*d>b*c){
        swap(a,c); swap(b,d);
    }
    // a/b < c/d
    // round(ai/b)==round(ci/d)==xごとに数える
    i128 A = a,B = b,C = c,D = d;
    // x==0
    i128 ans = (D + 2*C - 1)/(2*C);
    i128 E = (A*D + B*C)/(2*(B*C - D*A));
    ans += ceil_sum<i128>(E,2*C,2*D,3*D);
    ans -= ceil_sum<i128>(E,2*A,2*B,B);
    // i==0をcountしている
    ans--;
    cout << ans << "\n";
}