How can i write a Salesforce trigger to count number of completed task in a single contact?

I have completed this question till creating a task related with account.and count function is not completed,how can i do this.code is here

trigger copleteTask on Account (after insert)
{
List lst= new List();
for (account acc : trigger.new)
{
Task t = new Task();
t.status = 'complited';
t.WhatId = acc.id;

lst.add(t);
}

if (lst.size() > 0) {
insert lst;
}
}