import sys
input = lambda : sys.stdin.readline().rstrip()

write = lambda x: sys.stdout.write(x+"\n")
debug = lambda x: sys.stderr.write(x+"\n")
writef = lambda x: print("{:.12f}".format(x))


n = int(input())
val = n // (10**9+1)
ans = 0
b = len(str(val))
for k in range(1,b):
    ans += 9 * pow(10, (k+1)//2-1)
if b==1:
    ans = val
elif b%2==0:
    bb = b//2
    up = int(str(val)[:bb])
    ans += (up - pow(10, bb-1))
    if int(str(up) + str(up)[::-1])<=val:
        ans += 1
else:
    bb = b//2
    up = int(str(val)[:bb])
    ans += (up - pow(10, bb-1)) * 10
    for i in range(10):
        if int(str(up) + str(i) + str(up)[::-1])<=val:
            ans += 1
print(ans)