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) {
        Scanner sc=new Scanner(System.in);
        int a=sc.nextInt();
        int b=sc.nextInt();
        int t=sc.nextInt();
        int r=(int)((0.5*a*t)+(0.5*b*t));
        System.out.println(r);
    }
}