try:
  S=list(input())
except:
  print('NO')
  exit()
N=len(S)
for l in range(N+1):
  for r in range(l+1,N+1):
    if (r-l)==1 and r==N:
      continue
    X=set(S[:l]+S[r:])
    T=S[:]
    for i in range(N):
      if T[i] in X:
        T[i]=' '
    F=1
    for i in range(l,min(r+1,N)):
      if (i-r)&1==0:
        if T[i]!=' ':
          F=0
      else:
        if T[i]==' ':
          F=0
    if F:
      print('Yes')
      exit()
print('NO')