#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);
}