///Enta etfsh5t nseet el rank

#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("avx,avx2,fma")

#include "bits/stdc++.h"
#include <ext/pb_ds/assoc_container.hpp> // Common file
#include <ext/pb_ds/tree_policy.hpp> // Including tree_order_statistics_node_update

using namespace std;
using namespace __gnu_pbds;

template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag,
        tree_order_statistics_node_update>;
#define pb push_back
#define F first
#define S second
#define f(i, a, b) for (int i = a; i < b; i++)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define sz(x) (int)(x).size()
//#define mp(x, y) make_pair(x, y)
#define popCnt(x) (__builtin_popcountll(x))
#define int ll

using ll = long long;
using ull = unsigned long long;
using uint = uint32_t;
using ii = pair<int, int>;

const int N = 1e5 + 20, LG = 18, MOD = (119 << 23) + 1;
const long double PI = acos(-1);

const long double EPS = 1e-9;

int getLen(int x) {
    int ret = 0;
    while (x) {
        x /= 10;
        ret += 1;
    }
    return ret;
}

void doWork() {

    int a, b, c, d;
    cin >> a >> b >> c >> d;
    int ans = 0;
    do {
        a += 1;
        c += 1;
        ans += 1;
    } while (a % b != c % d);
    cout << ans << '\n';
}

int32_t main() {
#ifdef ONLINE_JUDGE
    ios_base::sync_with_stdio(0);
    cin.tie(0);
#endif // ONLINE_JUDGE
    int t = 1;
//    cin >> t;
    while (t--)
        doWork();
    return 0;
}