def ara_ara():
N={}
while 1:
src=input()
temp=src.split()
if len(temp)==1:
start=src
fin=input()
break
if N.get(temp[0]) is None:
N[temp[0]]=eval('["'+str(temp[1])+'"]')
else:
N[temp[0]] = eval(str(N.get(temp[0]))[0:-1]+',"'+temp[1]+'"]')
if N.get(temp[1]) is None:
N[temp[1]]=eval('["'+str(temp[0])+'"]')
else:
N[temp[1]] = eval(str(N.get(temp[1]))[0:-1]+',"'+temp[0]+'"]')
path=[]
q=[start]
while q:
v=q.pop()
if v not in path:
path = path +[v]
q=q+N[v]
if fin in path: return 'YES'
else: return 'NO'
if __name__ == '__main__':
print(ara_ara())
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