Collatz Conjecture: Why does my program go crazy after a certain number?

So I am playing around a bit with programming in my spare time and I decided to try writing a program that would use the collatz conjecture to decompose numbers according to certain rules. Long story short: if the number is even, divide it by two. if the number is odd, multiply it by 3 and add 1. So it works for all smaller numbers but if I hit a certain number (5 000 000 000 triggers it, but 1 000 000 000 doesn't, I am not sure what is the minimum number to trigger the bug). So when you enter a number too big the program goes crazy and spits out lots of numbers (some of them even go negative?) and it doesn't terminate. It just gets into an infinite loop. I am really curious why does it do that and what is the source of this behaviour. Keep in mind this is just me being curious and I don't actually have to do this to save my job or anything, I am doing it purely for fun so if you're a busy person don't waste time :). #include <iostream> using namespace std; void collatz(long unsigned int value); int main() { long unsigned int input; while (true) { cout << "Please enter a starting number(0 to terminate): "; cin >> input; if (input <= 0) { break; } collatz(input); cout << endl; } system("Pause"); return 0; } void collatz(long unsigned int value) { int steps = 0; long unsigned int l1 = 0, l2 = 0, l3 = 0; while (true) { steps++; if (l1 == 4 && l2 == 2 && l3 == 1) { cout << "Steps: " << steps << endl; break; } if (value % 2 == 0) { l1 = l2; l2 = l3; l3 = value; value = value / 2; cout << value << " - "; } else { l1 = l2; l2 = l3; l3 = value; value = value * 3; value = value + 1; cout << value << " - "; } } }