#include <stdio.h>
#include <string.h>

int main(argc,argv)
     int argc;
     char **argv;
{
  int n = 0;
  double avg;
  int flag = 0, t,  max[6000] , i, k, sum = 0 ;  
  FILE *fp;

  fp = fopen("file","rt");
  if(fp==NULL)	
    {
      printf("cannot open file");
      exit(0);	
    }
 
  while(!feof(fp))
    fscanf(fp,"%d", &max[n++]);		
  i = 0;
  while(i < n -1 && flag != 1)
    {
      if( max[i] == -1)
	flag = 1;
      else
	{// printf("\nmaxi = %d", max[i]);
	  sum += max[i];
	  i++;
	}
    }
  
  if(flag == 1)
    avg = -1.0;
  else
    avg = sum/(n-1);
  
  printf("%lf\n", avg);
}