#include <bits/stdc++.h>

#define pb push_back
#define mp make_pair
#define mt make_tuple
#define ll long long
#define pii pair<int,int>
#define tii tuple <int,int,int>
#define N 100005
#define mod 2000003
#define X first
#define Y second
#define eps 0.0000000001
#define all(x) x.begin(),x.end()
#define tot(x) x+1,x+n+1
using namespace std;
long double a, b, k1, k2, t, v1, v2;
int main() {
    cin.sync_with_stdio(0);
    cout.sync_with_stdio(0);
    cin >> a >> k1 >> b >> k2;
    v1 = 100000 / (a);
    v2 = 100000 / (b);
    cout << fixed << setprecision(7) << 100000 / (v1 * k1 + v2 * k2);
    return 0;
}