n, m = map(int, input().split())
s = list(map(int, input().split()))
for i in range(m):
    s[i] -= 1

st = set()
now = [x for x in range(n)]
for i in range(m):
    x, y = now[s[i]], now[s[i] + 1]
    st.add((min(x, y), max(x, y)))
    now[s[i]], now[s[i] + 1] = now[s[i] + 1], now[s[i]]

print(len(st))