N = int(input())
A = list(map(int, input().split())) 
cnt = 0
B = []
for i,a in enumerate(A,start=1):
    if a>N:
        B.append(a)
if len(B)==0:
    print(0)
    exit()
for i,b in enumerate(B,start=N+1):
    if b!=i:
        break
print(B[-1]-i+len(B))