I declared my variables as unsigned long long int and I have also changed the return type to this. It worked.

I just tried it, there are no issues.

it is IMPOSSIBLE to have 4000000000 as answer

It's a wrong assumption that long is always 32 bits. long is not less than int and not less than 32 bits. In 64-bit Linux it's 64 bits.

I tried to change the function to unsigned long but I get the same error.

You are probably converting to unsigned long when it's too late and the result is already truncated (at best; or even more likely undefined).

I tried to change the function to unsigned long but I get the same error.

And what's the message now? VS C++ uses 32 bit, but here it's 64 bit... and there are better ways to force 64 bits on different platforms? Many comments for a small solution of a simple kata. Fact is, there's no overflow here and no VS C++;-)!

It's a "long" which has 8 Byte or 64 Bit, try sizeof(m) which is 8... so there's no problem, because all the others are only int.