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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <iostream>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <cmath>
#include <algorithm>
#include <utility>
#include <functional>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
const int MOD = 1000000007;
const double EPS = 1e-9;


int main()
{
  cin.tie(0);
  ios::sync_with_stdio(false);

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

  return 0;
}