#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>

int main() {
  double x, y;
  double dx, dy;
  int nc, nq;
  int i;
  int N = 10000000;
  double R, R2;
  double r;

  srand48(time(NULL));
  
  R = 0.7;
  R2=R*R;
  r = 1000;
  
  nc = 0;
  nq = 0;
  x = 0;
  y = 0;
  
  for (i=0; i<N; i++) {
    dx = r*(2*drand48()-1);
    dy = r*(2*drand48()-1);
    if (fabs(x + dx)<1 && fabs(y+dy) < 1) { // se stanno nel quadrato
      x += dx;     // accetto la mossa
      y += dy;
    }
    if (x*x+y*y<R2) {
      nc ++;
    }
    nq ++;
  }
  printf("%f (%f)\n", 4.*nc/nq/R2, (double)nc/nq);
}