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

main(int argc, char **argv) {
  int id, N, np, i;
  double sum, left;

  if (argc != 4) {
    printf("Usage:\n%s id N np\n",argv[0]);
    exit(1);
  }

  id = atoi(argv[1]);
  N  = atoi(argv[2]);
  np = atoi(argv[3]);

  for(i = id, sum = 0; i < N; i += np) {
    double x = (i + 0.5) / N;
    sum += 4 / (1 + x * x);
  }

  sum /= N;
  printf("%lf", sum);
  exit(0);
}