#include <stdio.h>
#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000001


int main(){
	
	double X,Y,Z,W;
	cin>>X>>Y>>Z>>W;
	
	double ans = abs(X - Y*Z);
	ans /= 2.0;
	
	cout<<fixed<<setprecision(10)<<ans<<endl;
	
	return 0;
}