Complies but doesn't run

[Rayman452]

Hi, I'm completely new to programming. That being said, one of the programs I made for an assignment seems to compile, but when I run it I get an error. I can't figure out why its happening. Could someone point me in the right direction?

#include <iostream>
#include <string>
using namespace std;


int main()
{
    int number;
    
    cout<<"Please enter an integer greater than or equal to 1000: ";
    string digit;
    cin>>number;
    


if (number >= 1000)

    {
        string last_three_number = digit.substr(digit.length()-3);
        string first_three_number = digit.substr(0,3);
        cout<<"Your integer with commas is " <<first_three_number + "," + last_three_number <<endl;

    }

else if (number < 1000)

    {
        cout<<"Please enter a higher integer value"<<endl;
    }

return 0;


}