I have a very simple calculation where I calculate the distance between each individual cell to center in a 2D space. I know that the O(n) solution is redundant and I derived the formula for O(1) solution. But what I am trying to understand is: **Why do these two trivial calculations give me two different results?**

Here is the expected (correct) result in Python (Both versions give the same result):

```
result = 0;
n = 499993;
center = (n+1)//2;
for ii in range(1,center):
result += (ii*ii*8);
print(result);
```

which outputs:

```
41664916690999888
```

And here are two versions in C++ with two completely different wrong results:

1)

```
#include <iostream>
using namespace std;
int main()
{
unsigned long long result = 0;
int n = 499993;
int center = (n+1)/2;
for(int ii = 1; ii < center; ++ii)
{
result += (ii*ii);
}
cout << result*8 << endl;
}
```

Output:

```
154435732281936
```

```
#include <iostream>
using namespace std;
int main()
{
unsigned long long result = 0;
int n = 499993;
int center = (n+1)/2;
for(int ii = 1; ii < center; ++ii)
{
result += (ii*ii*8);
}
cout << result << endl;
}
```

Output:

```
6229295798864
```

**What is the reason of this behavior?**

For compiler I am using GCC with only -g flag

Online compiler for CPP that produces the same result: https://www.onlinegdb.com/online_c++_compiler

Thanks in advance!

Source: Windows Questions C++