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 in = new Scanner(System.in);
        int a = in.nextInt();
        int b = in.nextInt();
        int t = in.nextInt();
        System.out.println(growth(a,t)+growth(b,1));
    }
    
    
    private static int growth(int x, int t){
        return (int)(0.5*x*t);
    }
}