#include  <bits/stdc++.h>
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
using namespace std;

int main(void){
	int xa,ya,xb,yb;
	cin >> xa >> ya;
	cin >> xb >> yb;
	double a=1.0*(yb-ya)/(xb+xa);
	double b=1.0*ya+a*xa;
	cout.precision(10);
	cout << fixed << b << endl;
	return 0;
}