Javascript behind button error

Hi I am getting this error

Chrome - 'Unexpected token {'

Mozilla - 'missing ; before statement'

when clicking on a button, couldn't understand where I am going wrong. Please review my code below and guide me accordingly.

{!REQUIRESCRIPT("/soap/ajax/20.0/connection.js")}
{!REQUIRESCRIPT("/soap/ajax/22.0/apex.js")}
{!REQUIRESCRIPT("/support/console/31.0/integration.js")}

var CamId = '{!JSENCODE(Campaign.Id)}';

var result = sforce.apex.execute("CrtWlcomCallCampaign","welcomeCalls",{parentId:CamId});
sforce.debug.trace=true;

alert{'Welcome call campaign is initiated'}; 

here method welcomeCalls returns void.

Thanks in advance.