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<bits/stdc++.h>

#define fi first
#define se second
#define FO(x, n) for (int x = 0; x < n; ++x)
#define FOR(x, a, b) for (int x = a; x < b; ++x)
#define RFO(x, n) for (int x = n - 1; x >= 0; --x)
#define RFOR(x, a, b) for (int x = b - 1; x >= a; --x)
#define FOR_ITER(x, a) for(auto x = a.begin(); x != a.end(); ++x)



using namespace std;
typedef unsigned char byte;
typedef long long llong;
typedef unsigned long long ullong;
typedef pair<int, int> pii;
typedef pair<double, double> pdd;
typedef pair<string, string> pss;
typedef pair<llong, llong> pll;
inline bool feq(const double& a, const double& b) { return fabs(a - b) < 1e-10; }

using namespace std;

int n, k, x, y;

int main(){
//    int n, l;
//    cin >> n >> k;
//    vector<bool> dislike(10, false);
//
//    FOR(i, 0, k) {
//        int t;
//        cin >> t;
//        dislike[t] = true;
//    }

  //  string str;
    cin >> n >> k >> x;// >> y;
 //   cin >> str;
 //   vector<int> cnt(26, 0);


    cout << (double)(n + k) * x / 2 << endl;
    return 0;
}