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 calc = (double)(0.5 * a * t) + (double)(0.5 * b * t);
        int ans = (int) (calc);
        
        System.out.println(ans);
        
    }
}