#include <iostream>
#include <iomanip>
using namespace std;
int main()
{   int t1,t2,k1,k2;
    cin >> t1 >> k1 >> t2 >> k2;
    cout << setprecision(7) << (t1*t2)/(k1*t2+k2*t1+0.0) << '\n';
    return 0;
}