Arithmetic Progression Sum

Given the first two terms of an arithmetic progression and the number of terms, compute the sum of the progression.

An arithmetic progression is defined as: for any sequence \(a_1, a_2, \ldots, a_n\) if for every \(i \in [1, n)\) we have \(a_{i+1} - a_i = d\) (where \(d\) is a constant), then it is called an arithmetic progression. The sum of the first \(n\) terms is given by the formula: \(S = \frac{n}{2}(2a_1 + (n-1)d)\).

inputFormat

Input contains three space-separated integers representing the first term (a_1), the second term (a_2), and the number of terms (n) respectively.

outputFormat

Output the sum of the arithmetic progression.

sample

1 2 3

6