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 JavaScript (node.js v5.12) (judge verdict: Runtime Error)

using namespace std;

#include <iostream>
#include <cstdlib>
#include <algorithm>
#include <cmath>
#include <limits>
#include <climits>
#include <vector>
#include <string>
#include <set>
#include <queue>
#include <stack>
#include <map>
#include <numeric>
typedef long long ll;
typedef pair<int,int> p;

#define rep(i, n) for(int (i) = 0; (i) < (n); (i)++)


int main()
{
  int a,b,h;
  cin >> a >> b >> h;
  cout << (a+b) * h / 2 << endl;

}