Letter Frequencies in a String

You are given a string S composed of lowercase English letters. Your task is to compute the frequency of each letter and output the result as a concatenated string. The result should list each letter and its frequency in alphabetical order (i.e., increasing order by letter).

For example, given the input "apple", the frequencies of the letters are: a:1, e:1, l:1, p:2. Thus, the expected result is "a1e1l1p2".

Note: If the input string is empty, your program should output an empty string.

The frequency formula (if you like mathematical notation) can be represented as:

\( f(c) = \text{number of occurrences of } c \text{ in } S \)

inputFormat

The input consists of a single line containing the string S made up of lowercase English letters.

outputFormat

Output a single string that represents the frequency of each letter in S in alphabetical order, formatted as shown in the examples.

apple

a1e1l1p2