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 Rust (1.15.1) (judge verdict: Accepted)

#[allow(dead_code)]
fn scan<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

#[allow(dead_code)]
fn scan_split<T: std::str::FromStr>() -> Vec<T> {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().split_whitespace()
        .map(|t| t.parse().ok().unwrap()).collect()
}

#[allow(dead_code)]
fn scan_split_with<T: std::str::FromStr>(ch: char) -> Vec<T> {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().split(ch)
        .map(|t| t.parse().ok().unwrap()).collect()
}

#[allow(dead_code)]
fn scan_lines<T: std::str::FromStr>(n: usize) -> Vec<T> {
    (0..n).map(|_| scan::<T>()).collect()
}

#[allow(unused_macros)]
macro_rules! scan_as_tuple {
    ( $t:ty ) => {
        (scan::<$t>(),)
    };
    ( $( $t:ty ),* ) => {
        {
            let mut s = String::new();
            std::io::stdin().read_line(&mut s).ok();
            let v = s.trim().split_whitespace();
            let mut it = v;
            let ret = ( $(it.next().unwrap().parse::<$t>().ok().unwrap()),* );
            assert!(it.next() == None, "not all tokens consumed");
            ret
        }
    };
}

fn main() {
    let a = scan::<i32>();
    let b = scan::<i32>();
    let h = scan::<i32>();
    println!("{}", (a+b)*h/2);
}