05. Trapeziod Area

import java.util.Scanner;

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

        double a = Double.parseDouble(scanner.nextLine());
        double b = Double.parseDouble(scanner.nextLine());
        double h = Double.parseDouble(scanner.nextLine());

        double area = 0.5 * (a + b) * h;
        System.out.println(area);


    }
}