Hi guys, I'd like to ask for some help. When the input is a character or a word, it doesn't display "Invalid input"

int main()
{
    int userInput, gesture;

    cout << "Rock(0), Paper(1), Scissor(2): ";
    cin >> userInput;

    srand(time(0));
    gesture = rand() % 3;

    if (isdigit(userInput) && userInput >= 0 || userInput <= 2)        //I think it's a logic error, but idk how to fix it.
    {
        switch (gesture)
        {
        case (0):
            if (userInput == gesture)
                cout << "The computer is rock. You are rock too. It is a draw." << endl;
            else if (userInput == 1 && gesture == 0)
                cout << "The computer is rock. You are paper. You win." << endl;
            else if (userInput == 2 && gesture == 0)
                cout << "The computer is rock. You are scissor. You lose." << endl;
            break;
       
        }
    }

    else  

/* when the input is >2 or <0 it display the below statement, butwhen the input is an alphabet or a word, it doesn't reach here cout<<"Invalid input.";

    return 0;
}