import java.util.Scanner;

public class First {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        int b1 =Integer.parseInt(scanner.nextLine());
        int b2 = Integer.parseInt(scanner.nextLine());
        int h = Integer.parseInt(scanner.nextLine());
        double area=(((b1+b2)*h)*1.00)/2;
        System.out.printf("%.2f",area);
    }
}