#149. Formula: sum of the first n terms of an arithmetic progression
Find the sum of the first n terms of an arithmetic progression. Formula: `(2 * a1 + (n - 1) * d) * n // 2`
### Input
Each on its own line: a1, d, n (1 ≤ value ≤ 10).
### Output
One integer.
Examples
Input
9 8 3
Output
51
Input
10 9 1
Output
10
💡 You can solve it with Python, JavaScript or blocks. Read the input line by line with input(). Don't print a question (like input("Number: ")). Print exactly what the output asks for, nothing more.