Alice has a trapezium plastic. The plasticness of a plastic is the area of it. Given the length of the upper base, lower base, and the height of Alice's plastic, calculate the plasticness of it.

Input

The only line of input contains 3 integers: $b_u$, $b_l$ and $h$, representing the length of the upper base, lower base and the height of Alice's plastic.

Output

Output one integer: the plasticness of Alice's plastic.

Constraints

Subtask 1 (90%): $1 \le b_u, b_l, h \le 10000$, $h$ is even
Subtask 2 (10%): $1 \le b_u, b_l, h \le 10^9$, $h$ is even