极简题解|求1+2+3+…

有手就行

#include<iostream>
using namespace std;
unsigned long long co(unsigned long long a){
    unsigned long long b=a-1;
    if(b==1){
        return a+b;
    }else if(b<1){
        return a+b;
    }
    return a+co(b);
}
int main(){
    unsigned long long a;
    cin>>a;
    cout<<co(a);
}