function numero_de_news(){
for (i = 0; i < location.href.length; i++){
        if (location.href.substring(i, i + 9) == 'newsbrief'){
                return location.href.substring(i + 9, location.href.length - 5)
                }
        }
}

function format_char(numero){
	if (numero < 10)
		return ('000' + numero)
	if (numero < 100)
		return ('00' + numero)
	if (numero < 1000)
		return ('0' + numero)
}

function previous(){
	location.href = "newsbrief" + format_char(numero_de_news() - 1) + ".html"
}

function next(){
	location.href = "newsbrief" + format_char(numero_de_news() - (-1)) + ".html"
}