Problem:
Score : 100 points

Problem Statement

You are given a trapezoid. The lengths of its upper base, lower base, and height are a, b, and h, respectively.

An example of a trapezoid

Find the area of this trapezoid.

Constraints

1≦a≦100

1≦b≦100

1≦h≦100

All input values are integers.

h is even.

Input

The input is given from Standard Input in the following format:

a
b
h

Output

Print the area of the given trapezoid. It is guaranteed that the area is an integer.

Sample Input 1

3
4
2

Sample Output 1

7

When the lengths of the upper base, lower base, and height are 3, 4, and 2, respectively, the area of the trapezoid is (3+4)×2/2 = 7.

Sample Input 2

4
4
4

Sample Output 2

16

In this case, a parallelogram is given, which is also a trapezoid.

Solution in Perl (v5.18.2) (judge verdict: Wrong Answer)

#!/usr/bin/perl -w
 
chomp(@lines = <>);
%cards = (
    a => [split //, shift @lines],
    b => [split //, shift @lines],
    c => [split //, shift @lines]
);

my $player = 'a';
while (1) {
    my $card = shift @{$cards{$player}};
    print "$player:$card\n";
    if ($card) {

        $player = $card;
    } else {
        last;
    }
}
 
print uc $player;