#include "bits/stdc++.h"
#define int long long
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
const ll INF = (1LL << 61);
ll mod = 998244353;

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	int A, B, N, M; cin >> A >> B >> N >> M;
	if (A < B) {
		int tmp = M + 1;
		int cnt = (B - A) / tmp;
		int ans = max(min(A + cnt, B - cnt * M), min(A + cnt + 1, B - (cnt + 1) * M));
		cout << ans << endl;
	}
	else {
		swap(A, B);
		swap(N, M);
		int tmp = M + 1;
		int cnt = (B - A) / tmp;
		int ans = max(min(A + cnt, B - cnt * M), min(A + cnt + 1, B - (cnt + 1) * M));
		cout << ans << endl;
	}
	return 0;
}