server.get("/webhook/",function(req,res,next){
    var token = req.query.hub.verify_token;
    if( token === process.env.VALIDATION_TOKEN ){
        var challenge = parseInt( req.query.hub.challenge );
        res.send( req.query.hub.challenge );
    }else{
        res.send("Error, wrong validation token");
    }
    return next();

});