n, m = map(int, input().split())
s = list(map(int, input().split()))

g = [i for i in range(n+1)]
c = set()

for i in range(m):
    j =  s[i]
    c.add((g[j], g[j+1]))
    c.add((g[j+1], g[j]))
    g[j], g[j+1] = g[j+1], g[j]

print(len(c) // 2)