import java.io.*;
import java.util.*;

public class Solution {

    public static void main(String[] args) {
        /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
        Scanner in=new Scanner(System.in);
        int a=in.nextInt();
        int b=in.nextInt();
        int t=in.nextInt();
        double result;
        result=(0.5*a*t+0.5*b*t);
        int ans=(int)result;
        System.out.print(ans);
    }
}