#include<iostream>
#include<cstdio>
using namespace std;

int main()
{
    long long a,b,c;
    cin>>a>>b>>c;
    long long d=a+(a-b)*(c-1);
    long double ans=(a+d)*c/2;
    cout<<ans<<".00";
    return 0;
}
//全部WA