import sys

S=list(input())
A=[set() for i in range(26)]

x=S[0]
y=S[-1]
for i in range(len(S)):
    if S[i]==x or S[i]==y:
        S[i]=" "
    
        

for i in range(len(S)):
    if S[i]==" ":
        continue
    A[ord(S[i])-97].add(i%2)

if len(S)<=2:
    print("NO")
    sys.exit()
    

for a in A:
    if len(a)==1:
        print("Yes")
        break
else:
    print("NO")