#include <bits/stdc++.h>
using namespace std;
#define int long long int
#define double long double
#define print(a) for(auto x : a) cout << x << " "; cout << endl
const int M = 1000000007;
const int N = 3e5+9;
const int INF = 2e9+1;
const int LINF = 2000000000000000001;
inline int power(int a, int b, int mod=M) {
int x = 1;
a %= mod;
while (b) {
if (b & 1) x = (x * a) % mod;
a = (a * a) % mod;
b >>= 1;
}
return x;
}
//_ ***************************** START Below *******************************
double consistency(int a, int b, int c, int d){
double ans = a*d;
ans /= (b*d - (b-a)*(d-c));
return ans;
}
double practice(int a, int b, int c, int d){
return 0;
}
void solve() {
int a, b, c, d;
cin >> a >> b >> c >> d;
cout << fixed << setprecision(7) << consistency(a, b, c, d) << endl;
}
int32_t main() {
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int t = 1;
// cin >> t;
while (t--) {
solve();
}
return 0;
}
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