int hit=0;
int miss=0;

void setup() {
  size(500,600);
  background(255);
  noFill();
  ellipse(width/2, width/2, width, width);
  fill(255);
  strokeWeight(2);
  textAlign(LEFT,CENTER);
  textSize(48);
}

void draw() {
  float x = random(width);
  float y = random(width);  // stay within a square
  if (dist(x,y,width/2,width/2) <= width/2) {
    hit++;
    stroke(0, 255, 0);
  } else {
    miss++;
    stroke(255, 0, 0);
  }
  point(x,y);
  noStroke();
  fill(204);
  rect(0, 1+width, width, height-width);
  float estimate = 4.0 * hit / (hit+miss);
  fill(0);
  text(nf(estimate, 1, 8), width/4, width + 0.5*(height-width));
}