function check_correct()
{
	sum=document.getElementById('sum').value;
    period=document.getElementById('period').value;
    submit=document.getElementById('calc_it');
    if (sum<=1000000 && sum>=1 && period>=1 && period<=12)
	{
        submit.disabled=false;
        return true;
    }
    submit.disabled=true;
    return false;
}