N,M = map(int,input().split())
S = [0]+list(map(int,input().split()))
A = list(range(N+1))
ans = set()
for i in range(1,M+1):
    s = S[i]
    l = S[i]
    r = S[i]+1
    a = [A[l],A[r]]
    a = sorted(a)
    ans.add(tuple(a))
    A[l],A[r] = A[r],A[l]
print(len(ans))