#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, e) for (int i = (int)s; i < (int)e; ++i)
#define all(a) (a).begin(),(a).end()

int main() {
    cin.tie(nullptr);
    
    double xa, ya, xb, yb;
    cin >> xa >> ya >> xb >> yb;
    
    xb *= -1;
    
    double ans = -((ya - yb) / (xa - xb)) * xa + ya;
    
    cout << fixed << setprecision(15) << ans << '\n';
}