That's because memory on the stack is limited. There's much more on the heap.
On windows, you could try (assuming mingw uses gnu gcc compiler )
Go to CodeBlocks->Settings->Compiler and Debugger->Linker settings->Other linker options and add
"--stack,16777216" (no quotes) to increase stack size to 16MB (or use any other reasonable value)
Then your code should work, unless you want 20 million booleans.


not that stack size is different from the total memory available to the program
(you probably have at least a gigabyte of memory available on your own computer.)

Or you could've just written your program in one of the following ways.

Make a global boolean array of size 5 million

or
#include <vector>
...
vector<bool> isPrime (5000000); //(may cause frustration in some cases)
or
bool*isPrime = new bool[5000000];
...
delete []isPrime;

Yeah, executing your program on the terminal gives
"terminate called after throwing an instance of 'std::bad_alloc'
  what():  std::bad_alloc
Aborted"
However, even when the memory limitation is removed, it TLEs.

You can do the math yourself.

The memory limit is 16MB - that's 16,777,216 bytes.
A boolean is 1 byte, an integer is 2 bytes, and a longint is 4 bytes.

In your case it means memory access out of bounds.
What happens in your code if M or N is greater than 5000000?

In your particular case it means you are using too much memory.

but the judge says that i'm only using 64kb of memory for that one

The memory reading is often inaccurate, and also since your attempt at allocating a huge array fails, the memory usage doesn't include the size of that array anyways.

What can you replace cin or cout with?

scanf and printf

no it's 2 seconds per test case, so my program could thereotically take 10 seconds.  however, I can't take 3 secs on 1 test case and 1 on another.

Actually, looking at Sour Spinach's code, he still used the same algorithm. But not in the same way as the rest of us.

¿Cómo? Did you mean "dissent"?