N,M = map(int, input().split())
S = list(map(int, input().split()))
PATTERN = set()
L = list(range(N))
for s in S:
    PATTERN.add((min(L[s-1],L[s]),max(L[s-1],L[s])))
    L[s-1],L[s]=L[s],L[s-1]
print(len(PATTERN))