#include<bits/stdc++.h>
using namespace std;
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    double x,y,z,w;
    cin>>x>>y>>z>>w;
    double ans = (x/2)-(y*z/2);
    cout<<fixed<<setprecision(7)<<ans<<endl;
}