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


int main() {
    
    long long int t,r;
    
    int a,b;
    
    cin>>a>>b>>t;
    
    r=((0.5)*a+(0.5)*b)*t;
    
    cout<<r;
    
    return 0;
}