Problems
Recursion - 2
Recursion - 2
The integers a, b, c are given. Implement the recursive function:
f(n) = \begin{cases} a, n = 0\\f(n - 1) + bn + c, n > 0\end{cases}
Input data
Four integers a, b, c\:(|a|, |b|, |c| \le 1000), n\:(0 \le n \le 1000).
Output data
Print the value of f(n).
Examples
Input example #1
4 2 -3 10
Output example #1
84