#include <iostream>
int hansoo(int n){ // 1000 미만
if(n >= 100){
int sum = 0;
int n1, n2, n3;
for(int i = 100; i <= n; i++){
n1 = i / 100;
n2 = (i - n1 * 100) / 10;
n3 = i % 10;
if((n1 - n2) == (n2 - n3)){
sum += 1;
}
}
return 99 + sum;
} else {
return n;
}
}
int main(int argc, char **argv){
int N; // 1~1000
int sum;
scanf("%d", &N);
printf("%d", hansoo(N));
return 0;
}