def pairs(k, arr):
    # Convert the list to a set for O(1) lookups
    arr_set = set(arr)
    count = 0
    
    # Check for each element if there's a pair with difference k
    for x in arr:
        if x + k in arr_set:
            count += 1
        if x - k in arr_set:
            count += 1
    
    # Since each pair is counted twice, we return half the count
    return count // 2