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 (GCC 5.4.1) (judge verdict: Runtime Error)

#include<stdio.h>

int main(){
		char s[3][101];
		scanf("%s%s%s",s[0],s[1],s[2]);
		int si[3]={0},gen[3]={0},t=0,i;
		while(s[0][gen[0]]!=0)gen[0]++;
		while(s[1][gen[1]]!=0)gen[1]++;
		while(s[2][gen[2]]!=0)gen[2]++;
		while(1){
				//for(i=0;i<3;i++){
						if(si[t]==gen[t]){
								printf("%c\n",'A'+i);
								return 0;
						}
				//}
				t=s[t][si[t]++]-'a';				
		}
}