#include<iostream>
using namespace std;

int main()
{
	int k1,k2,t1,t2;
	cout<<"t1=";cin>>t1;
	cout<<"t2=";cin>>t2;
	cout<<"k1=";cin>>k1;
	cout<<"k2=";cin>>k2;
	cout<<k1/(float)t1+k2/(float)t2;
}