#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <math.h>
#include <cmath>
#include <limits.h>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <functional>
#include <stdio.h>
using namespace std;

long long MOD = 1000000007;

int main() {
    
    
    double xa,ya,xb,yb;
    cin >> xa >> ya >> xb >> yb;
    
    xa = -xa;
    
    printf( "%.7f", ya - ((ya-yb)/(xa-xb))*xa );
    
    return 0;
}