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)

fn main() {
    let a = read_i64();
    let b = read_i64();
    let h = read_i64();
    println!("{}", solve(a, b, h));
}

fn solve(a: i64, b: i64, h: i64) -> i64 {
    (a + b) * h / 2
}

fn read_i64() -> i64 {
    let mut buf = String::new();
    std::io::stdin().read_line(&mut buf);
    buf.trim().parse().unwrap()
}

fn read_vec_i64() -> Vec<i64> {
    use std::str::FromStr;
    let mut buf = String::new();
    std::io::stdin().read_line(&mut buf).unwrap();
    buf.trim().split(' ').map(|s| i64::from_str(s).unwrap()).collect()
}