#include <bits/stdc++.h>
using namespace std;

//int Yuki0236()
int main()
{
int i;
long long A;
long long B;
long long X;
long long Y;

	cin >> A;
	cin >> B;
	cin >> X;
	cin >> Y;

	i = A * Y - B * X;
	if( i > 0 )
		printf( "%.7f\n", (long double) ( B * X ) / A + X );
	else if( i < 0 )
		printf( "%.7f\n", (long double) ( A * Y ) / B + Y );
	else
		cout << "Err" << endl;

	return 0;
}