#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;
    cin >> a >> b >> t;    
        
    double noVir = 0.5 * a * t + 0.5 * b * t;     
    
    std::cout << noVir;
    
    return 0;
}