#include <bits/stdc++.h>
// clang-format off
using namespace std; using ll=long long; using ull=unsigned long long; using pll=pair<ll,ll>; const ll INF=4e18;
void print0(){}; template<typename H,typename... T> void print0(H h,T... t){cout<<h;print0(t...);}
void print(){print0("\n");}; template<typename H,typename... T>void print(H h,T... t){print0(h);if(sizeof...(T)>0)print0(" ");print(t...);}
void perr0(){}; template<typename H,typename... T> void perr0(H h,T... t){cerr<<h;perr0(t...);}
void perr(){perr0("\n");}; template<typename H,typename... T>void perr(H h,T... t){perr0(h);if(sizeof...(T)>0)perr0(" ");perr(t...);}
void ioinit() { cout<<fixed<<setprecision(15); cerr<<fixed<<setprecision(6); ios_base::sync_with_stdio(0); cin.tie(0); }
#define debug1(a) { cerr<<#a<<":"<<a<<endl; }
#define debug2(a,b) { cerr<<#a<<":"<<a<<" "<<#b<<":"<<b<<endl; }
#define debug3(a,b,c) { cerr<<#a<<":"<<a<<" "<<#b<<":"<<b<<" "<<#c<<":"<<c<<endl; }
#define debug4(a,b,c,d) { cerr<<#a<<":"<<a<<" "<<#b<<":"<<b<<" "<<#c<<":"<<c<<" "<<#d<<":"<<d<<endl; }
// clang-format on

ll extgcd(ll a, ll b, ll& x, ll& y) {
    if (b == 0) {
        x = 1;
        y = 0;
        return a;
    }
    ll d = extgcd(b, a % b, y, x);
    y -= a / b * x;
    return d;
}
ll gcdf(ll a, ll b) {
    if (b == 0) {
        return a;
    }
    return gcdf(b, a % b);
}

int main() {
    ioinit();
    ll b0, c0, b1, c1;
    cin >> b0 >> c0 >> b1 >> c1;
    c0 %= b0;
    if (c0 < 0) c0 += b0;
    c1 %= b1;
    if (c1 < 0) c1 += b1;
    ll g = gcdf(b0, b1);
    if (c0 > c1) {
        swap(b0, b1);
        swap(c0, c1);
    }
    ll dif = c1 - c0;
    if (dif % g != 0) {
        print("NaN");
        return 0;
    }
    ll x, y;
    extgcd(b0, b1, x, y);
    y = -y;

    x *= dif / g;
    y *= dif / g;
    {
        ll s = b0 * x + c0;
        ll t = b1 * y + c1;
        // s + (b1/g)*p >= 0
        // t + (b0/g)*p >= 0
        ll p = 0;
        if (s < 0) {
            ll d1 = b1 / g;
            p = max(p, (d1 - 1 - s) / d1);
        }
        if (t < 0) {
            ll d0 = b0 / g;
            p = max(p, (d0 - 1 - t) / d0);
        }
        x += p * b1 / g;
        y += p * b0 / g;
    }

    {
        ll s = b0 * x + c0;
        ll t = b1 * y + c1;
        print(s);
    }

    return 0;
}