Help with extracting characters from string

[valentino1337]

For our project we need to extract single characters from a given string which are separated from each other by the increasing number of places. Each extracted character must be exactly one place more apart from its predecessor than the one before. We need to use a string formed from the extracted characters as an input for the next phase of the project. Does anyone know how to solve this.

Example:
Input string: "A clown has a red nose.".
Output string: "A lnsee".