#include "bits/stdc++.h"
using namespace std;

#define REP(i, n) for(int i=0; i<(n); i++)

signed main()
{
    int A,B,X,Y;
    cin>>A>>B>>X>>Y;
    double coffee = min(1.0 * X, 1.0 * Y * A / B);
    printf("%.14f\n", coffee * (1.0 + 1.0 * B / A));
    return 0;
}