為什么我在C ++中不斷得到錯誤的輸出

Question

int main()
{
int n, i, sum;

cout << "Enter a value for n: ";
cin >> n;
sum = 0;

    i = 1;
    do
{
    sum += i * i;
    i++;
}while (i <= n);
cout << "The sum of the first " << n
    << " numbers is " << sum << endl;

return 0;
}

每當我運行此代碼並使用4作為示例時，輸出結果為30。4的階乘為24時

Answer 1

sum+=i*i 

->前n平方的和。

階乘是

prod=prod*i