if(scanf("%d",&x1)!=1)
    {
        printf("Incorrect input");
        return 1;
    }
    printf("Podaj x2: ");
    if(scanf("%d",&x2)!=1)
    {
        printf("Incorrect input");
        return 1;
    }
    if(x1>x2)
    {
        printf("Incorrect input");
        return 1;
    }
    if(x1==x2)
    {
        printf("Nothing to show");
        return 0;
    }
    for(i=x1;i<=x2;i++)
    {
        if(is_twin(i,i+2)==1&&(i+2)<=x2)
        {
            printf("%d %d\n",i,i+2);
            licznik++;
        }
    }
    if(k==0)
    {
        printf("Nothing to show");
    }