01.3.01TrapeziodArea

import java.util.Scanner;

public class ex01TrapeziodArea {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        double b1 = Double.parseDouble(scan.nextLine());
        double b2 = Double.parseDouble(scan.nextLine());
        double h = Double.parseDouble(scan.nextLine());

        double square = (b1 + b2) * h / 2;

        System.out.printf("%.2f", square);

    }
}