Using python write the function most common substring(S,m) that receives a string S and an integer m and returns the substring of length m that appears the most times in S and the number of times that substring appears. Your solution must use a dictionary and run in time O(n).

S= ACCAAGGGTTTTGTAGGTGTATACATACCAGGGCAATCGGCAACACGTCTGGTGGAGGTGTGCATAGAACTGCAGTGAGTGTTCGATGTGTGTATTGATAA

n= 1 through 7