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

float tinhsopi(long n){
 float s = (float) 2/3;

 for(long i=2;i<=n;i++)
    if (i%2==0) s= s +  (float) 1/ (float) (2*i+1);
     else s = s - (float) 1/ (float) (2*i+1);

  return(s);
 }

void main(){

  long n;
  printf("\n nhap n=");
  scanf("%ld",&n);

  float sopi = (float) tinhsopi(n) *4;

  printf("\n so pi la %3.3f", sopi);

  getch();
  }