题解 | #等差数列#

#include<stdio.h>

int main() { int n = 0;

while (scanf("%d", &n) != EOF)
{
	int tmp = 2;
	int sum = 0;
	while (n--)
	{
		sum += tmp;
		tmp += 3;
	}
	printf("%d\n", sum);

}

return 0;

}