file = open('17.txt')
text = file.readline()
d = dict
Max = 0
for i in range(len(text) - 2):
    if text[i - 2] == text[i]:
        for c in text:
            if c in d:
                d[c] += 1
                Max = max(Max, d[c])
            else:
                d[c] = 1
ans = []
for c in d:
    if d[c] == Max:
        ans.append(c)
ans.sort()
print(ans[0])