#include<fstream>
#include<cstring>
#include<queue>
#include<algorithm>
#include<string.h>
#include<bitset>
#include<ctime>
#include<set>
#include<cmath>
#include<unordered_map>
#include<iomanip>
#include<map>
#include<stack>
#include<vector>
#include<bitset>
#include<functional>
#include<iostream>
#include<deque>
#include<unordered_set>
#include<queue>
#include<list>
#include<array>
#include<ctype.h>
using namespace std;
long long i, j, k, x, y, z, ok, nr, n, m, el, min1, max1;
double t1,t2,k1,k2;
int main()
{
	cin >> t1 >> k1 >> t2 >> k2;
	cout << 1/(1/(t1 / k1) + 1/(t2 / k2));
	return 0;
}