#include<bits/stdc++.h>

using namespace std;
int human[5]; 
int year[5];
int times[5];	
int v,tot;
double z;
int main(){
	
	cin>>human[1];
	cin>>year[1];
	cin>>human[2]>>year[2];
	
	times[1]=human[1]*year[1];
	times[2]=human[2]*year[2];
	
	z=abs(times[1]-times[2])/abs(year[1]-year[2]);
	
	cout<<fixed<<setprecision(2);
	cout<<z;
	
	return 0;
}