#include <bits/stdc++.h>

using namespace std;

double x;
int t1 , k1 , t2 , k2;

int main()
{

//freopen("test.in" , "r" , stdin);
//freopen("test.out" , "w" , stdout);

cin >> t1 >> k1 >> t2 >> k2;

x = (1.0 * t1 / k1 * t2 / k2) / (1.0 * t1 / k1 + 1.0 * t2 / k2);
cout << fixed << setprecision(9) << x << '\n';

return 0;
}