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)

using namespace std;
#include<bits/stdc++.h>
#define BEGIN ios_base::sync_with_stdio(0);cin.tie(0);
#define END return EXIT_SUCCESS;
#define FOR(I,A,B) for((I)=(A);(I)<(B);(I)++)
#define REP(I,N) FOR((I),0,(N))
#define UP(I,A,B) for((I)=(A);(I)<=(B);(I)++)
#define DW(I,A,B) for((I)=(A);(I)>=(B);(I)--)
#define IN(P) cin>>(P)
#define IN2(P1,P2) cin>>(P1)>>(P2)
#define IN3(P1,P2,P3) cin>>(P1)>>(P2)>>(P3)
#define INS(I,N,V) REP((I),(N)) cin>>(V[I])
#define INS2(I,N,V1,V2) REP((I),(N)) cin>>(V1[I])>>(V2[I]);
#define INS3(I,N,V1,V2,V3) REP((I),(N)) cin>>(V1[I])>>(V2[I])>>(V3[I]);
#define OUT(P) cout<<(P)<<endl
#define OUT2(P1,P2) cout<<(P1)<<" "<<(P2)<<endl
#define OUT3(P1,P2,P3) cout<<(P1)<<" "<<(P2)<<" "<<(P3)<<endl
#define OUTS(I,N,V) REP((I),(N)) cout<<(V[i])<<endl
#define ALL(C) (C).begin(),(C).end()
#define RALL(C) (C).rbegin(),(C).rend()
#define mp make_pair
#define pb push_back
#define lb lower_bound
#define ub upper_bound
template<class T>inline bool chmax(T& a,T b){if(a<b){a=b;return true;}return false;}
template<class T>inline bool chmin(T& a,T b){if(a>b){a=b;return true;}return false;}

int a,b,h;
inline void solve(){
  IN3(a,b,h);
  OUT((a+b)*h/2);
}

int main(int argc,char**argv){
  BEGIN
  solve();
  END
}