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: Runtime Error)

#!perl
use strict;
use warnings;
use utf8;
use feature qw/say/;

my $S_a = <STDIN>;
my $S_b = <STDIN>;
my $S_c = <STDIN>;

chomp $S_a;
chomp $S_b;
chomp $S_c;

my $cards = +{};
$cards->{a} = [split(//, $S_a)];
$cards->{b} = [split(//, $S_b)];
$cards->{c} = [split(//, $S_c)];

my $current_player = 'a';
while ( scalar @{ $cards->{$current_player} } > 0) {
    $current_player = shift $cards->{$current_player};
}

say $current_player;