shabi选手刚学OI，问个问题

n=int(input())
a=[0]*(n+5)
fac=[0]*(n+5)
fac[0]=1
fre=0
dlen=0
for i in range(1,n+1):
	fac[i]=fac[i-1]*i
for i in range(1,n+1):
	a[i]=int(input())
	if a[i]==-1:
		fre=fre+1
	else:
		dlen=dlen+a[i]-1
if dlen>n-2:
	print("0")
	exit()
ans=fac[n-2]//fac[n-2-dlen]
for i in range(1,n+1):
	if a[i]!=-1:
		ans=ans//fac[a[i]-1]
for i in range(0,n-2-dlen):
	ans=ans*fre
print(int(ans))
exit()