#include <stdio.h>
double pi(double a)
{
int i=1;
double t=1.0,p=0.0;
while(i/(2*i+1)>=a)
{
p=p+t;
t*=i/(2*i+1);
i++;
}
return 2*p;
}
main()
{
double p,a;
scanf("%lf",&a);
p=pi(a);
printf("%lf",p);
return 0;
}
public class Pi
{
public static void main(String args[]){
double sum = 0;
double flag = 1.0;
for (long i=1;i<1000000000 ;i++ )
{
sum += flag/(2*i-1);
flag = -flag;
}
double pi = 4*sum;
System.out.println(pi);
}
};
{
public static void main(String args[]){
double sum = 0;
double flag = 1.0;
for (long i=1;i<1000000000 ;i++ )
{
sum += flag/(2*i-1);
flag = -flag;
}
double pi = 4*sum;
System.out.println(pi);
}
};
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