#include<stdio.h>
int main(){
  int a,b,c;
  double sum;
  a=1,sum=0,c=1;
  scanf("%d",&b);
  while(c<=b){
      if(c%2==0){
        sum=sum-c/a;}
    else{
        sum=sum+c/a;}
        a=a+2,c=c+1;
  }
  printf("%.3f",sum);
  return 0;
}

#include<stdio.h>
int main(){
  int x,a,g,c;
  double sum,average;
  a=1,c=0,sum=0,average=0;
  scanf("%d",&x);
  if(x==0){
      printf("average = 0.0\n");
  }
  else {
    while(a<=x){
    scanf("%d",&g);
    if(g>=60){
      c=c+1;
    }sum=sum+g;
    a=a+1;
  }
  average=sum/x;
  printf("average = %.1f\n",average);}
  printf("count = %d",c);
  return 0;
}