Simple. Iterate through the array once, and add each number to a dictionary. Then, iterate through a second time, and, for each number, see if that number + k exists in the library. If it does, then you know you have a pair, and can increment anwser

def pairs(a, k)
    d = {}
    answer = 0
    for i in a:
        d[i] = i
    for j in a:
        g = j+k
        if g in d:
            answer +=1
    return answer