d=dict()
while True:
try:
a=input()
for i in a:
if ord('a') <= ord(i) <=ord('z'):
if i in d:
d[i]+=1
else:
d[i]=1
except:
break
L = list(d.items())
K = max(d.values())
L.sort(key = lambda t: t[0])
for i in L:
if i[1] == K:
print(i[0],end="")