#define _USE_MATH_DEFINES
#include<stdio.h>
#include<string>
#include<iostream>
#include<cctype>
#include<cstdio>
#include<vector>
#include<stack>
#include <algorithm>
#include<math.h>
#include<set>
#include<map>
#include<iomanip>

//#include<bits/stdc++.h>



using namespace std;




int main() {
	
	double a,b,c,d;
	cin>>a>>b>>c>>d;
	
	double ans=0;
	if(b*c>=a*d)ans=d+d*a/b;
	else ans=c+c*b/a;
	cout<< setprecision(30);
	cout<<ans<<endl;


	return 0;
}