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 in=new Scanner(System.in);
        int a,b,t;
        a=in.nextInt();
        b=in.nextInt();
        t=in.nextInt();
        double d=a*t*0.5;
        double e=b*t*0.5;
        int y,u;
        u=(int) (d);
        y=(int)(e);
        System.out.println(u+y);
        
        
    }
}