import java.io.*;
import java.util.*;

public class Solution {

    public static void main(String[] args) {
       Scanner scan=new Scanner(System.in);
         int i=scan.nextInt();
           int j=scan.nextInt();
           int k=scan.nextInt();
         
        double s=(double)0.5*i*k+(double)0.5*j*k;
         
         System.out.println((int)s);
    }
}