var canvas=document.getElementById("demo");
var cxt=canvas.getContext("2d");
function circle()
{
canvas.onmousedown=function(evt)
{
cxt.lineWidth=1;
evt=window.event||evt;
var startX=evt.pageX-this.offsetLeft;
var startY=evt.pageY-this.offsetTop;
}
canvas.onmousemove=function(evt)
{
var x=evt.pageX-this.offsetLeft;
var y=evt.pageY-this.offsetTop;
cxt.strokeStyle="rgb(180,0,0)";
cxt.beginPath();
var radii=Math.sqrt((startX-x)*(startX-x)+(startY-y)*(startY-y));
cxt.arc(startX,startY,radii,0,Math.PI*2,false);
cxt.stroke();
cxt.closePath();
}
}