import sys
input = sys.stdin.readline

G, P = int(input()), int(input())
ans = 0
parents = [i for i in range(G+1)]

def find(x) :
    if x != parents[x] :
        parents[x] = find(parents[x])
    return parents[x]

def union(x, y) :
    x = find(x)
    y = find(y)

    parents[x] = y


for _ in range(P) :
    g = int(input())
    gate = find(g)
    
    if gate==0 :
        break

    union(gate, gate-1)
    ans += 1

print(ans)