function valid_form(f){
	var error = "";
	if (f.nom.value=="" || f.nom.lenght<1) { 
		error += "Your name is required.\n";
	}
	if (f.prenom.value=="" || f.prenom.lenght<1) { 
		error += "Your first name is required.\n";
	}
	if (f.tel.value=="" || f.tel.lenght<1) { 
		error += "Your phone number is required.\n";
	}
	if (f.mail.value=="" || f.mail.lenght<1) { 
		error += "Your email is required.\n";
	}
	
	if(error != ""){
		alert(error);
		return false;
	}else{
		return true;
	}
}