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())