Num 5

rep_str="bbaaa"
k=3
found = False
tmp = rep_str[0]*k
for i in range(len(rep_str)):
    if tmp in rep_str:
        found = True
        print "The first substring of length " + str(k) +" is "+ str(tmp) +"!"
        break
    tmp = rep_str[i]*k

if not found:
    print "nothing found for the string " + str(rep_str) +"!"