题解

#include<iostream>
using namespace std;
int main(){
	long long n;
	cin>>n;
    if(n<6){
        cout<<n;
    }else{
        cout<<5+(1+n-5)*(n-5)/2;//（首项+末项）*项数/2
    }
    return 0;
}

#include<iostream>
using namespace std;
int main(){
	long long n,sum=5;
    cin>>n;
    for(int i=1;i<n-4;i++){
        sum+=i;
    }
    cout<<sum;
    return 0;
}