my_string = ""
k = 5
current_len = 0
letter = my_string[0]
succeded = False

for i in range(0, len(my_string)):
    if my_string[i] == letter:
        current_len += 1

    else:
        letter = my_string[i]
        current_len = 1

    if current_len == k:
        print(letter * k)
        succeded = True

if not succeded:
    print("Didnt find a substring of length %d "% k)