import sys
input = sys.stdin.readline
N, M = map(int, input().split())
S = list(map(int, input().split()))
ans = set()
dp = [i for i in range(N)]
for s in S:
    s-=1
    a, b = dp[s], dp[s+1]
    if a>b:
        a, b = b, a
    ans.add((a, b))
    dp[s], dp[s+1] = dp[s+1], dp[s]
print(len(ans))