N, M = map(int, input().split())
S = list(map(lambda x: int(x) - 1, input().split()))

ans = set()
seats = [i for i in range(N)]
for s in S:
    # s, s+1 のポジションを交換
    u, v = seats[s], seats[s + 1]
    u, v = min(u, v), max(u, v)
    ans.add((u, v))
    seats[s], seats[s + 1] = seats[s + 1], seats[s]

print(len(ans))