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Background

Description

梯形的上边为a，下边为b，高为h，求梯形的面积。

Format

Input

第一行输入上边a（正整数1<=a<=1000）

第二行输入下边b（正整数1<=b<=1000）

第三行输入高h（正整数1<=h<=1000）

Output

输出梯形的面积（无须考虑保留的位数）

Samples

10
20
30

450

2
3
5

12.5

Limitation

1s, 1024KiB for each test case.