// #include <bits/stdc++.h>
#include <iostream>
#include <vector>
#include <string>
#include <set>
#include <map>
#include <queue>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll,ll> PP;
// #define MOD 1000000007
#define MOD 998244353
#define INF 2305843009213693951ll
#define PI 3.141592653589
#define setdouble setprecision
#define REP(i,n) for(ll i=0;i<(n);++i)
#define OREP(i,n) for(ll i=1;i<=(n);++i)
#define RREP(i,n) for(ll i=(n)-1;i>=0;--i)
#define ORREP(i,n) for(ll i=(n);i>=1;--i)
#define rep(i,a,b) for(ll i=(a);i<=(b);++i)
#define ALL(v) (v).begin(), (v).end()
#define chmin(k,m) k = min(k,m)
#define chmax(k,m) k = max(k,m)
#define GOODBYE do { cout << "-1" << endl; return 0; } while (false)
#define MM <<" "<<
#define Endl endl

__int128_t floor_sum(__int128_t N, __int128_t M, __int128_t A, __int128_t B){
    // Copyright (c) 2025 0214sh7
    // https://github.com/0214sh7/library/

    __int128_t Ans = 0;
    if(A >= M){
        Ans += (N*(N-1)/2) * (A/M);
        A %= M;
    }
    if(B >= M){
        Ans += N * (B/M);
        B %= M;
    }

    if(A==0){
        Ans += N*(B/M);
        return Ans;
    }

    __int128_t K = (A*(N-1)+B)/M;
    Ans += N*K - floor_sum(K,A,M,M-B+A-1);

    return Ans;

}

int main(void){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    
    ll A,B,C,D;
    cin >> A >> B >> C >> D;
    if(A*D == C*B){
        cout << -1 << endl;
        return 0;
    }
    if(A*D > C*B){
        swap(A,C);
        swap(B,D);
    }

    // A/B < C/D
    __int128_t N = B*D/(B*C - A*D);
    __int128_t Ans = N;
    Ans -= floor_sum(N+1,2*D,2*C,D);
    Ans += floor_sum(N+1,2*B,2*A,B);
    cout << (long long)Ans << endl;
    
    return 0;
    
}