4050: CF122 - B. Lucky Substring
[Creator : ]
Description
Petya loves lucky numbers. Everybody knows that lucky numbers are positive integers whose decimal representation contains only the lucky digits 4 and 7. For example, numbers 47,744,4 are lucky and 5,17,467 are not.
One day Petya was delivered a strings, containing only digits. He needs to find a string that
- represents a lucky number without leading zeroes,
- is not empty,
- is contained insas a substring the maximum number of times.
The lexicographical comparison of strings is performed by the < operator in the modern programming languages. String $x$ is lexicographically less than string $y$ either if $x$ is a prefix of $y$, or exists such $i\ (1≤i≤min(|x|,|y|))$, that $x_i<y_i$ and for an $y_j\ (1≤j<i)\ x_j=y_j$. Here $|a|$ denotes the length of string $a$.
Input
The single line contains a non-empty string $s$ whose length can range from 1 to 50, inclusive. The string only contains digits. The string can contain leading zeroes.
Output
In the only line print the answer to Petya's problem. If the sought string does not exist, print "-1" (without quotes).
Sample 1 Input
047
Sample 1 Output
4
three conditions are fulfilled for strings "4", "7" and "47". The lexicographically minimum one is "4".
Sample 2 Input
16
Sample 2 Output
-1
s has no substrings which are lucky numbers.
Sample 3 Input
472747
Sample 3 Output
7
the three conditions are only fulfilled for string "7".