[BaekJoon/Python] 1065번 한수

n=int(input())
hansu=0

for i in range(1,n+1):
    if i <100:
        hansu+=1
    else:
        n_list=list(map(int,str(i)))
        if n_list[2]-n_list[1]==n_list[1]-n_list[0]:
            hansu+=1
        
print(hansu)