The program that you enter in the do while statement, it works once, but then it's weird.

int main(void)
{
    char ch;
    int x, y;
    int result;

    printf("*****************\n");
    printf("A--- Add\n");
    printf("S--- Subtract\n");
    printf("M--- Multiply\n");
    printf("D--- Divide\n");
    printf("Q--- Quit\n");
    printf("*****************\n");

    do
    {
    repeat:
        printf("Select operation:");
        //ch = getchar();
        while (1) {
            ch = getchar();

            if (ch != '\n')
                break;
        }

        printf("Enter two numbers separated by a space: ");
        scanf_s("%d %d", &x, &y);

        if (ch == 'A')
            result = x + y;
        else if (ch == 'S')
            result = x - y;
        else if (ch == 'M')
            result = x * y;
        else if (ch == 'D')
            result = x / y;
        else if (ch == 'Q')
            break;
        else
            goto repeat;

        printf("%d\n", result);


    } } while (1);

    return 0;
}