#include<stdio.h>
#include<stdlib.h>
#include<math.h>
int main ()
{
  int N;
  int* v=NULL;
  float p=0;
  scanf("%d",&N);
  printf("\n");
  v=malloc(N*sizeof(int));
  for(int i=0;i<N;i++)
  {
    scanf("%d",&v[i]);
    p=(v[i]+v[i+1])/2;
    i++;
    p=(p+v[i])/2;
  }
  printf("\n%f",p);
  return 0;

}
