C++ problem with long string input (10^6 characters long) with no whitespaces [closed]

So I have to input a very long string which contains only two characters like "a" or "b" for example, and has no white spaces or " \n". I know how long is string going to be. I just need to know on which places are "a" and on which "b".

When input is like 3000 characters long everything works fine, but when i want to input a string of (for example) 900000 characters, program isn't responding.

Do you have any idea how to approach this problem?

(Sorry for my bad English)