#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 */  
    unsigned  int a,b,t,count;
    scanf("%d %d %d",&a,&b,&t);
    count=(.5*a*t+.5*b*t);
    printf("%d",count);
    
    return 0;
}