题解 | 等差数列
#include <stdio.h>
int main() {
int n, num, sum;
while (scanf("%d", &n) != EOF) {
num = 2;
for(int i = 0; i < n;i++)
{
sum += num;
num += 3;
}
printf("%d\n", sum);
}
return 0;
}

