#include
#include
#include
int main()
{
double pi,t,n,s;
s=t=1;
pi=0;
n=1;
while((fabs(t))>=1.0E-6)
{
pi=pi+t;
n=n+2;
s=-s;
t=s/n;
}
pi=pi*4;
printf( "%.6lf\n",pi);
return 0;
}