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: Wrong Answer)

#include <stdio.h>
int main(void)
{
  int high_len,low_len,height,size;
  char str[4];
  fgets(str,sizeof(str),stdin);
  sscanf(str,"%d",&high_len);
  fgets(str,sizeof(str),stdin);
  sscanf(str,"%d",&low_len);
  fgets(str,sizeof(str),stdin);
  sscanf(str,"%d",&height);
  size = (high_len + low_len) * height / 2;
  printf("%d\n",size);
  return 0;
}