function check_input(input, msg_box)
{
    if((input.value == '') || isNaN(input.value))
    {
        document.getElementById(msg_box).innerHTML = 'Vous devez saisir un nombre valide (ex: 790.5)';
        return false;
    }
    
    document.getElementById(msg_box).innerHTML = '';
    return input.value;
}

function valid_form(form, msg_box)
{
    check_input(document.getElementById('montant_pret'), 'msg_montant_pret');
    check_input(document.getElementById('taux_interet'), 'msg_taux_interet');
    check_input(document.getElementById('taux_assurance'), 'msg_taux_assurance');
    check_input(document.getElementById('duree_pret'), 'msg_duree_pret');
    
    if(
        check_input(document.getElementById('montant_pret'), 'msg_montant_pret') && 
        check_input(document.getElementById('taux_interet'), 'msg_taux_interet') && 
        check_input(document.getElementById('taux_assurance'), 'msg_taux_assurance') && 
        check_input(document.getElementById('duree_pret'), 'msg_duree_pret')
        )
    {
        var cap	= parseFloat(check_input(document.getElementById('montant_pret'), 'msg_montant_pret'));
        var tx = parseFloat(check_input(document.getElementById('taux_interet'), 'msg_taux_interet'));
        var assur = parseFloat(check_input(document.getElementById('taux_assurance'), 'msg_taux_assurance'));
        var n_mens = parseFloat(check_input(document.getElementById('duree_pret'), 'msg_duree_pret'));
        
        // Taux + assurance
        tx =tx+assur;

        m=cap*(tx/100)/(1-Math.pow((1+tx/100/12),(-n_mens)))/12;
        m=Math.floor(m*100)/100;

        // Coût du crédits
        total = n_mens * m;
        total = parseInt(total - cap); 

        document.getElementById(msg_box).innerHTML = "Vos mensualit&eacute;s seront de&nbsp;:<br /><span>"+m+" &euro;</span><br />Le co&ucirc;t total de votre cr&eacute;dit sera de&nbsp;:<br /><span class=\"cout\">"+total+" &euro;</span>";
    }
    
    else document.getElementById(msg_box).innerHTML = '';
}