#include <iostream> #include <fstream> #include <cstring> #include <stdlib.h> #include <stdio.h> #include <math.h> #include <vector> #include <stack> #include <list> #include <algorithm> #include <limits.h> using namespace std; int main(){; int t1, k1, t2, k2; cin >> t1 >> k1 >> t2 >> k2; double res = (double)t1 * t2 /(k1 * t2 + k2 * t1); cout << res << endl; }