华为oj等差数列

#include<iostream>
using namespace std;

int getSum(int n)
{
int start = 2;
int step = 3;
int sum = 0;
int last = start+(n-1)*step;
sum = (start+last)*n/2;
return sum;

}
int main()
{
int n;
cin>>n;
if(n>0)
cout<<getSum(n)<<endl;
else
return -1;
return 0;
}