题解

#include<bits/stdc++.h>
using namespace std;
int main(){
    int n,ans = 0;
    cin>>n;
    for(int i = 1;i <= n;i++){
        int k = i;
        while(k > 0){
            if(k % 10 == 1) ans++;
            k /= 10;
        }
    }
    cout<<ans;
    return 0;
}