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


int main() {
    int a, b, t;
    cin >> a >> b >> t;
    int d = (a+b)*t/2;
    cout << d;
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    return 0;
}