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

#define N 99

double myarctan(double x)
{
int i;
int sign;
double at = 0.0;

sign = 1;
for (i = 1; i <= N; i++) {
if (i%2 == 0) continue;
at += sign * pow(x, i)/i;
sign *= -1;
}
return at;
}

int main(void)
{
double pi;
pi = 4*(myarctan(1/2) + myarctan(1/4) + myarctan(1/8));

printf("π=%g\n", pi);
}