import sys
input = sys.stdin.readline

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

for i in range(M):
    A[i]-=1

ANS=list(range(N))

SET=set()

for i in range(M):
    x=A[i]
    y=A[i]+1

    k,l=ANS[x],ANS[y]

    if k<l:
        SET.add((k,l))
    else:
        SET.add((l,k))

    ANS[x],ANS[y]=ANS[y],ANS[x]

print(len(SET))