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

int a[20], b[20], c[20], r[20], max[6000] , max1[6000];    

int main(argc,argv)
     int argc;
     char **argv;
{
  int n = 0;
  int flag = 0, t, k, temp, i, j, c1;
  double min ;

  FILE *fp;
  
  int q = 0;
  
 
  fp = fopen(argv[1],"rt");
  if(fp==NULL)	
    {
      printf("cannot open file");
      exit(0);	
    }
 
  while(!feof(fp))
    fscanf(fp,"%d", &max[n++]);		
  
  for (i = 0; i < 5000; i++)
    for(j=i+1; j< 5000; j++)
    {
     if(max[j] < max[i])
       { temp = max[i];
	 max[i] = max[j];
         max[j] = temp;
       }
    }
  k = max[n-2];
 
/*  while( i < 5000)
    {
      printf("\nmaxi = %d", max[i]);
      i++;
      }*/
  

  
  n = 1;
  max1[0] = 0;
  fp = fopen(argv[2],"rt");
  
  if(fp==NULL)	
    {
      printf("cannot open file");
      exit(0);	
    }
  
  while(!feof(fp))
    fscanf(fp,"%d", &max1[n++]);
  
  // printf("\nmaxn2 = %d", max1[n-2]);
  
  i = 1;
  j = 2;
  
  while( i < n - 2 && flag != 1)
    { 
      while(j< n -2 && flag != 1)
	{
	  if(max1[j] > max1[i])
	    {
	      flag = 0;
	      j++;
	    }
	  else
	    flag = 1;
	}  
      i++;
    }
  
  if(flag == 0)
    {
      c1 = max1[n-2];
      i = 2;
      while( i < n -2 && flag != 1)
	{
	  t = max1[i] - max1[i-1];
	  if ( 5000/k <= t && t <= (5000/k + 1))
	    i++;
	  else
	    flag = 1;
	}
      
      if(flag == 0)
        printf("%d\n", c1);
      else
	printf("-1\n");
    }
  
  else
    printf("-1\n");
}