#include <stdio.h>
int main()
{
int N, ans=0, N1=0, N2=0, N3=0;
scanf_s("%d", &N);
if (N < 100)
{
ans = N;
}
else if (N >= 100 && N <= 1000)
{
ans = 99;
for (int i = 100; i <= N; i++)
{
N1 = i / 100;
N2 = (i - (N1 * 100)) / 10;
N3 = i - (N1 * 100 + N2 * 10);
if ((N3 - N2) == (N2 - N1))
{
ans++;
}
}
}
printf("%d", ans);
return 0;
}