1049 Counting Ones

#include<iostream>
using namespace std;

int a[]={1,10,100,1000,10000,100000,1000000,10000000,100000000,1000000000};
int b[]={0,1, 20, 300, 4000, 50000, 600000, 7000000, 80000000, 900000000};

int countone(int n)
{
    if(!n)return 0;
    int i,j;
    for(i=0;n/a[i]>9;i++);
    j=n/a[i];
    if(j==1){
        return b[i]+1+countone(n-a[i])+n-a[i];
    }
    else{
        return b[i]*j+a[i]+countone(n-j*a[i]);
    }
}

int main()
{
    int N;
    cin>>N;
    cout<<countone(N);
}
//4321
//0-999-n
//1000-1999 1000+n
//2000-2999 n
//3000-3999 n
//4000
//4000-4321
//0001-0321
//0-9999 n*10+1000

//1300
//0-999
//1000
//1001-1300