#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;
const double eps = 1e-12;
int main()
{
int n, m;
double p, q, lambda;
while (cin >> n >> m >> p >> q)
{
if (n == 0)
{
cout << "0.00" << endl;
continue;
}
if (m == 0)
{
cout << "1.00" << endl;
continue;
}
else if (p == 0.0 || q == 1.0)
{
cout << "0.00" << endl;
continue;
}
lambda = q * (1 - p) / p / (1 - q);
if (fabs(lambda - 1.0) < eps)
cout << fixed << setprecision(2) << double(n) / (n + m) << endl;
else
cout << fixed << setprecision(2) << (1 - pow(lambda, n)) / (1 - pow(lambda, n + m)) << endl;
}
return 0;
}
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