题解

#include<bits/stdc++.h>
using namespace std;
string x(string s1,string s2){
	int a[10010]={},b[10010]={},c[10010]={};
	int l1 = s1.size(),l2 = s2.size();
	for(int i=0;i<l1;i++) a[l1-i-1] = s1[i]-'0';
	for(int i=0;i<l2;i++) b[l2-i-1] = s2[i]-'0';
	for(int i=0;i<l1;i++){
		for(int j=0;j<l2;j++){
			c[i+j] += a[i]*b[j];
			c[i+j+1] += c[i+j]/10;
			c[i+j]%=10;
		}
	}
	int l = 0;
	for(int i=l1+l2;i>=0;i--){
		if(c[i]){
			l = i;
			break;
		}
	}
	stringstream str__;
	for(int i=l;~i;i--) str__<<c[i];
	return str__.str();
}
string add(string as,string bs,int n){
	char t[37];
	int a[10010]={},b[10010]={},c[10010]={};
	for(int i=0;i<37;i++) t[i]=i>9 ? i-10+'A' : i+'0';
	for(int i=0;i<as.size();i++)a[as.size()-1-i]=isdigit(as[i]) ? as[i]-'0' : as[i]-'A'+10;
	for(int i=0;i<bs.size();i++)b[bs.size()-1-i]=isdigit(bs[i]) ? bs[i]-'0' : bs[i]-'A'+10;
	int l=max(as.size(),bs.size());
	for(int i=0;i<l;i++){
		c[i]+=a[i]+b[i];
		c[i+1]+=c[i]/n;
		c[i]%=n;
	}	
	c[l]?l++:l=l+0;
	stringstream str;
	for(int i=l-1;~i;i--)str<<t[c[i]];
	return str.str();
}
int main(){
	int n=0;
	cin>>n;
	string last="1",sum="0";
	for(int j=1;j<=n;j++){
		last="1";
		for(int i=1;i<=j;i++){
			stringstream str____;
			str____.str()="";
			str____<<i;
			last=x(last,str____.str());
		}
		sum=add(sum,last,10);
	}
	cout<<sum;
}