function countKeys(k, s) {
  var c = 0;

  for (var l = 0, m = s + 1; l < m; l++)
    c += countSubstringsL(k, s, l);

  return c;
}
Function returning the minimum number of substrings that will be selected by
the multi-match-aware selection scheme for theshold `k`, for a string of
length `s` to match any string of relevant length.

@param   {number} k - Levenshtein distance threshold.
@param   {number} s - Length of target strings.
@returns {number}   - The number of selected substrings.