title: How to calculate no. of binary strings containg substring "00"?
body: <p>I need to calculate no of possible substrings containing "00" as a substring.
  I know the length of the binary string. </p>
  
  <p>Eg: for a string of length 4, possible substrings are:
  0000
  0001
  0010
  0011
  0100
  1000
  1001
  1100</p>
  
  <p>I just need the number of possible combinations, not enumerate all of them </p>
  
  <p>PS: max length 10^6</p>