s= 'aaaarytartugaasstsyssss'
d = dict()
Max = 0
for c in s:
    if c in d:
        d[c] += 1
        Max = max(Max, d[c])
    else:
        d[c] = 1
a = []
for c in d:
    if d[c] == Max:
        a.append(c)
a.sort()
print(a[0])