一次性AC，用时5分钟

#include<iostream>
#include<iomanip>
using namespace std;
int sum(int n1,int n2)
{
    int he=0;
    for(int i=n1;i<=n2;i++)
        he+=i;
    return he;
}
int main()
{
//  freopen("xxx.in","r",stdin);
//  freopen("yyy.out","w",stdout);
    int n1,n2;
    cin >> n1 >> n2;
    cout << sum(n1,n2);
//  fclose(stdin); 
//  fclose(stdout);
    return 0;
}