#include <bits/stdc++.h>
using namespace std;
#define ios ios_base::sync_with_stdio(false);cin.tie(0);
#define setnow clock_t tStart=clock();
#define time (double)(clock() - tStart)/CLOCKS_PER_SEC;
//#define setin(x) ifstream cin(x);
//#define setout(x) ofstream cout(x);
typedef long long ll;
typedef long long int lli;
typedef pair < int, int> dbl;
const int maxInt = 1e9*2;
const lli maxLong = 1e18*2;
int main(){
			double t1, t2, k1, k2;
			cin >> t1 >> k1 >> t2 >> k2;
			t1/=k1;
			t2/=k2;
			cout << fixed << setprecision(10) << (double)(t1*t2)/(t1+t2);
}