N, M = map(int, input().split())
A = list(map(int, input().split()))
Aset = set(A)
if M%2:
	print(0)
	exit()
ans = 0
i = 0
while A[i] < M//2:
	if (A[i] + M//2) % M in Aset:
		ans += M-2
	i += 1

print(ans)