N, M = map(int, input().split())
S = list(map(int, input().split()))

A = list(range(N))
SET = set()
for s in S:
    s -= 1
    SET.add((min(A[s], A[s+1]), max(A[s], A[s+1])))
    A[s], A[s+1] = A[s+1], A[s]

print(len(SET))