def generate_my_string(string_size, substring_size, distinct_chars):

   my_string = ""
   count_1 = 0
   count_2 = 0

   for i in range (string_size):
      count_1 += 1
      count_2 += 1

      if (count_1 <= substring_size):
         if (count_2 <= distinct_chars):
            my_string = my_string + chr(96 + count_1)

         else:
            my_string = my_string + 'a'

      else:
         count_1 = 1
         count_2 = 1
         my_string = my_string + 'a'

   return my_string

my_string_size = 8
my_substring_size = 6
K_distinct_chars = 4

print("The string size is :")
print(my_string_size)

print("The substring size is :")
print(my_substring_size)

print("The distinct characters count is :")
print(K_distinct_chars)

print("The resultant string is :")
print(generate_my_string(my_string_size, my_substring_size, K_distinct_chars))

The string size is :
8
The substring size is :
6
The distinct characters count is :
4
The resultant string is :
abcdaaab