#include<bits/stdc++.h>
typedef long long ll;
using namespace std;
int INF = 1e9;
int MOD = 1e9+7;
main(){
    int xa,ya,xb,yb;
    cin >> xa >> ya >> xb >> yb;
    if(xa==xb && ya==yb){
        cout << ya << endl;
        return 0;
    }
    printf("%.6lf\n",yb - 1.*(yb-ya)/(xb+xa)*xb);
}