#include <iostream>
#include <cstdio>
#include <cstring>
#include <iomanip>
#include <algorithm>
//#include <vector>
//#include <cmath>
//#include <stack>
//#include <bitset>
//#ifndef LOCAL
//#define cerr dolor_sit_amet
//#endif
//#include <queue>
#define mp make_pair
//#define sz(x) ((int)((x).size()))
#define X first
#define Y second
using namespace std;


int main()
{
    double t1,k1,t2,k2;
    cin>>t1>>k1>>t2>>k2;
    cout<<(t1*t2/(k1*k2))/(((t1/k1)+(t2/k2)));
    return 0;
}