Subarray Sum Equals k

Given an array of integers nums and an integer k, your task is to find the total number of continuous subarrays whose sum equals to k. A subarray is a contiguous part of the array.

The problem can be formulated using prefix sums. Let \( prefix[i] \) denote the sum of the first \( i \) elements. A subarray from index \( j \) to \( i-1 \) has sum equal to \( k \) if:

\[ prefix[i] - prefix[j] = k \]

Your solution should efficiently count all such subarrays.

inputFormat

The input is given via standard input (stdin) with the following format:

• The first line contains two space-separated integers: n (the number of elements in the array) and k (the target sum).
• The second line contains n space-separated integers representing the elements of the array.

outputFormat

Output a single integer to standard output (stdout) which is the number of continuous subarrays whose sum equals to k.

## sample

8 7
4 3 2 1 1 1 1 1

3