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 a=new Scanner(System.in);
       
        int factor_1=a.nextInt();
        int factor_2=a.nextInt();
        int t=a.nextInt();
        double answer=(0.5*factor_1*t)+(0.5*factor_2*t);
        System.out.println((int)answer);
        
    }
}