#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    int ans;
    int a,b;
    double ans_a=0,ans_b=0,last=0;
    
    long sec;
    cin>>a>>b>>sec;
    for(long i=1;i<=sec;i++)
    {
        ans_a = 0.5 * a * (double)i;
        ans_b = 0.5 * b * (double)i;
        
        last += ans_a + ans_b;       
    }
    cout<<last;
    return 0;
}