# C++、C、Python三语言题解
首先，$\Sigma_{i=1}^{n}i^2=n(n+1)(2n+1)/6$，这一点可以通过数论证明。然后套公式即可，时间复杂度$O(1)$
## C++语言
```cpp
#include <iostream>
using namespace std;
int main()
{
int a, b;
cin>>a>>b;
int r = a * (a + 1) * (2 * a + 1) / 6;
cout<<r%b<<endl;
return 0;
}
```
## C语言
```c
#include <stdio.h>
int main()
{
int a, b;
scanf("%d%d", &a, &b);
int r = a * (a + 1) * (2 * a + 1) / 6;
printf("%d", r%b);
return 0;
}
```
## Python
```python
a=int(input())
print(a*(a+1)*(2*a+1)//6%int(input()))
```