#include <stdio.h>
int main() {
    int n;
    double sum=0,sum1=0;
    int count=0;
    scanf("%d",&n);
    for(int i=1;i<=2*n-1;i+=2)
    {
        if(count%2==0)
         sum+=i;
         else
          sum-=i;;
          sum1+=1.0/sum;
          count++;
    }
      printf("%.3f",sum1);
    return 0;
}