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;
}