How to find missing lines in a file in python or c++?
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I have a big file in which every line begins with a number. I have around 4000 lines and they all begin with a number in order. Like this :
1: bla bla bla
2: the cat is mine
3: programming issue
and so on until 4000. 
How can i found out which lines are missing related to the number? How to implement that in a script so that if line no. 2134 is missing, it would print : "missing lines are :..." ?
Thanks!