TrapezoidArea

import java.util.Scanner;

public class TrapeziodArea {
    public static void main(String[] agrs) {

        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 area=(b1+b2)*h/2;
        System.out.println(area);

    }
}