#include <bits/stdc++.h>
#define rep(i, a, n) for(int i = a; i < n; i++)
#define repb(i, a, b) for(int i = a; i >= b; i--)
#define all(a) a.begin(), a.end()
#define o(a) cout << a << endl
#define int long long
#define fi first
#define se second
using namespace std;
typedef pair<int, int> P;

signed main(){
    int b[4];
    rep(i, 1, 4) cin >> b[i];
    int ans = (b[3] * b[3] - b[3] * b[2] + b[2] * b[2] - b[3] * b[1]) / (b[2] - b[1]);
    cout << ans << endl;
}