def k(n):
	def k(n):
		if n==1:
			for i in range(1,10):
				yield str(i)
			return
		l=n&1
		m=n//2
		for i in range(10**(m-1),10**m):
			s=str(i)
			if l:
				for j in range(10):
					yield s+str(j)+s[::-1]
			else:
				yield s+s[::-1]
	for i in range(1,18):
		for j in k(i):
			l=len(j)
			t=j+"0"*(9-l)+j[::-1]
			t=int(t)
			if t>n:break
			yield t
n=int(input())
#for i in k(n):print(i,i%(10**9+1))
print(sum(i<n for i in k(n)))