25pts!求调！！！

#include<iostream>
#include<string>
#include<algorithm>
using namespace std;
struct bigNum{
    string num;
    bigNum()
    {
        num.clear();
    }
    bigNum operator +(const bigNum num2)const{
        bigNum ans;
        string s1=num;
        string s2=num2.num;
        int len1=s1.size();
        int len2=s2.size();
        reverse(s1.begin(),s1.end());
        reverse(s2.begin(),s2.end());

        int lenn=max(len1,len2);
        int x=0;
        for(int i=0;i<lenn;i++){
            if(i>=len1) s1+='0';
            if(i>=len2) s2+='0';
            ans.num+=(((s1[i]-'0')+(s2[i]-'0')+x)%10+'0');
            x=((s1[i]-'0')+(s2[i]-'0')+x)/10;
        }
        //cout<<s1<<" "<<s2<<" "<<ans.num<<endl;
        if(x==1){
            ans.num+=(x+'0');
        }
        reverse(ans.num.begin(),ans.num.end());
        return ans;
    }
    bigNum operator - (const bigNum num2)const{
        bigNum ans;
        string s1=num;
        string s2=num2.num;
        if(s1.size()<s2.size()||(s1.size()==s2.size()&&s1<s2)){
            swap(s1,s2);
            cout<<"-";

        }
        int len1=s1.size();
        int len2=s2.size();
        reverse(s1.begin(),s1.end());
        reverse(s2.begin(),s2.end());
        int lenn=max(len1,len2);
        int x=0;
        for(int i=0;i<lenn;i++){
            if(i>=len1) s1+='0';
            if(i>=len2) s2+='0';
            if(s1[i]<s2[i]||s1[i]+x<'0'){
                ans.num+=(s1[i]-'0'+10)-(s2[i]-'0')+x+'0';
                x=-1;
            }else{
                ans.num+=(s1[i]-'0')-(s2[i]-'0')+x+'0';
                x=0;
            }
        }
        //cout<<s1<<" "<<s2<<" "<<ans.num<<endl;

        while(ans.num[lenn-1]=='0'&&lenn>1){

            ans.num.erase(lenn-1);
            lenn--;
        }
        reverse(ans.num.begin(),ans.num.end());
        return ans;
    }
    bigNum operator * (const bigNum num2)const{
        bigNum ans;
        string s1=num;
        string s2=num2.num;
        int len1=s1.size();
        int len2=s2.size();
        reverse(s1.begin(),s1.end());
        reverse(s2.begin(),s2.end());

        int lenn=len1+len2;
        while(ans.num.size()<lenn) ans.num+='0';
        for(int i=0;i<len1;i++){
            int x=0;
            for(int j=0;j<len2;j++){
                //if(i+j>=ans.num.size()){
                //    ans.num+=(((s1[i]-'0')*(s2[j]-'0')+x))%10+'0';
                //}else{
                    ans.num[i+j]=(ans.num[i+j]-'0'+((s1[i]-'0')*(s2[j]-'0')+x))%10+'0';
                //}
                x=((s1[i]-'0')*(s2[j]-'0')+x)/10;
            }

            ans.num[i+len2]=x+'0';
        }
        //cout<<s1<<" "<<s2<<" "<<ans.num<<endl;
        //cout<<ans.num.size()<<endl;
        while(ans.num[lenn-1]=='0'&&lenn>1){

            ans.num.erase(lenn-1);
            lenn--;
        }
        reverse(ans.num.begin(),ans.num.end());
        return ans;
    }
};
int main(){
    int n;
    cin>>n;
    bigNum ans;
    ans.num="0";
    for(int i=1;i<=n;i++){
        bigNum k;
        k.num="1";
        for(int j=1;j<=i;j++){
            bigNum a;
            a.num=to_string(j);
            k=k*a;
        }
        ans=ans+k;
    }
    cout<<ans.num;
    return 0;
}