Button click fires when I refresh page

I have a button on form.I have implemented button_click event for the button as follows

protected void button_Click(object sender,EventArgs e)
{
//i did following operation

// update salarytbl set salary=salary+1000
}

Here if i click button it will fire click event and update salary say salary is 500 then updated will be 1500

but now when i click refresh button again salary is updated to 2500, again i click refresh salary is 3500.. it goes

ie button click is fired when i say refresh.

why it is happening??