Find missing sequence example in Python

 def find_missing(sequence): #[1,3,4,6,7,8]  
   result = []  
   if len(sequence)>2:  
     a = min(sequence[1] - sequence[0],sequence[2] - sequence[1]) #1   
   #sequence = str(sequence)  
   for i in range(0,len(sequence)):  
     if sequence[i+1] - sequence[i] != a:  
       return sequence[i]+ a  
   return -1  
 find_missing([1,2])