#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 a,b,t,h=0;
    cin>>a>>b>>t;
     
      
   h=0.5*a*t+0.5*b*t;
    
    cout<<h;
    
    return 0;
}