#include<stdio.h>
#include<stdlib.h>
double PAI(int n)
{
double pai;
if(n==1)
{
pai=2/3;
return 2/3;
}
else
{
pai=PAI(n-1)+1/(4*n-3)-1/(4n-1);
return pai;
}
int main()
{
int n;
scanf("%d",&n);
printf("%lf",PAI(n));
return 0;
}Yexiaomo
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