#include <iostream> #include <algorithm> #include <cmath> #include <vector> #define mp make_pair #define pb push_pack #define LL long long #define inf 2e9 #define zeros x^(x-1)&x using namespace std; double t1,k1,t2,k2; int main() { cin>>t1>>k1>>t2>>k2; t1 /= k1; t2 /= k2; cout<<t1*t2 / (t1+t2); return 0; }