PAT乙级题 1006 python解答

num=str(input())
index=0
list=[]
last=''
for x in num:
list.append(x)
index+=1
n=int(list[len(list)-1])
for c in range(1, n + 1):
last=last+(str(c))
if index==3:
string='B'*int(list[0])+'S'*int(list[1])+last
elif index==2:
string='S'*int(list[0])+last
else:
string=last
print(string)