import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

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 scan=new Scanner(System.in);
        int a=scan.nextInt();
        int b=scan.nextInt();
        int t=scan.nextInt();
        
        double ans=(0.5*t)*(a+b);
        int g=(int)ans;
        System.out.println(g);
    }
}