N,K = map(int,input().split())
A = list(map(int,input().split()))
sample = [n for n in range(N+1)]
tmp = set()
for k in range(K):
    i = A[k]
    tmp.add((min(sample[i],sample[i+1]),max(sample[i],sample[i+1])))
    sample[i],sample[i+1]  = sample[i+1],sample[i]
print(len(tmp))