package programmingBasics;

import java.util.Scanner;

public class TrapeziodArea {
    public static void main(String[] args) {
        Scanner scanner= new Scanner(System.in);
        int b =Integer.parseInt(scanner.nextLine());
        int b1=Integer.parseInt(scanner.nextLine());
        int h =Integer.parseInt(scanner.nextLine());
        int area=(b+b1)*h/2;
        System.out.printf("%.2d",area));
    }
}