int cx, cy, r;

void setup() {
  size(300,300);
  background(255);
  
  ellipseMode(RADIUS);
  cx = 150;
  cy = 150;
  r  = 30;
}

void draw() {
  clearToWhite();
  stroke(0);
  fill(255);
  ellipse(cx, cy, r, r);
}

void clearToWhite() {
  fill(255,70);
  noStroke();
  rect(0,0,width,height);
}

void mouseDragged() {
  float d = sqrt((cx - mouseX) * (cx - mouseX)
               + (cy - mouseY) * (cy - mouseY));
  if( d < r ) {
    cx = mouseX;
    cy = mouseY;
  }
}