>>184
#include <stdio.h>

int EnNoNaka(double x, double y) { return (x * x + y * y <= 1)? 1: 0; }
int main(void)
{
int n;
double x, y, d;
int count = 0;

printf("input n >> "); scanf("%d", &n);

d = 1.0 / (double)n;
for (x = -1; x < 1; x += d) {
for (y = -1; y < 1; y += d) {
if (EnNoNaka(x, y) && EnNoNaka(x + d, y + d)) count++;
}
}

printf("Answer = %lf\n", count * d * d);
return 0;
}