Vanya and Books - 洛谷 | 计算机科学教育新生态 (luogu.com.cn)

ac代码

#include<bits/stdc++.h>
typedef long long ll;

#define IOS ios::sync_with_stdio(0),cin.tie(0),cout.tie(0)
const ll N=1e3;
using namespace std;

int main()
{
    IOS;
    ll x;
    cin>>x;
    ll ans=0,p=x;
    int k2=0,k=0;
    while(x)
    {
        k++;
        x/=10;
    }
    k2=k;
    ll a=pow(10,k-1);
    if(k2==k) ans+=k*(p-a+1);
    //cout<<ans<<endl;
    for(int i=1;i<k;i++)
    {
        ans+=i*9*pow(10,i-1);
        //cout<<ans<<endl;
    }
    cout<<ans<<endl;
    return 0;
}