Problem:
Score : 400 points

Problem Statement

We have a string S of length N consisting of R, G, and B.

Find the number of triples (i,~j,~k)~(1 \leq i < j < k \leq N) that satisfy both of the following conditions:

S_i \neq S_j, S_i \neq S_k, and S_j \neq S_k.

j - i \neq k - j.

Constraints

1 \leq N \leq 4000

S is a string of length N consisting of R, G, and B.

Input

Input is given from Standard Input in the following format:

N
S

Output

Print the number of triplets in question.

Sample Input 1

4
RRGB

Sample Output 1

1

Only the triplet (1,~3,~4) satisfies both conditions. The triplet (2,~3,~4) satisfies the first condition but not the second, so it does not count.

Sample Input 2

39
RBRBGRBGGBBRRGBBRRRBGGBRBGBRBGBRBBBGBBB

Sample Output 2

1800

Solution in Perl (5.26.1) (judge verdict: Accepted)

use v5.26; # strict say state biwise
use warnings;
use List::Util qw(reduce any all none notall first max maxstr min minstr product sum sum0 pairs unpairs pairkeys pairvalues pairfirst pairgrep pairmap shuffle uniq uniqnum uniqstr);

chomp (my $n = <STDIN>);
chomp (my $s = <STDIN>);
my ($ans, $r, $g, $b) = (0, 0, 0, 0);
for my $i (0..$n-1) {
  if (substr($s, $i, 1) eq 'R') {
    $r++;
  } elsif (substr($s, $i, 1) eq 'G') {
    $g++;
  } else {
    $b++;
  }
  for my $j ($i+1..$n-1) {
    my $k = $j + $j - $i;
    last if $k >= $n;
    $ans++ if substr($s, $i, 1) ne substr($s, $j, 1) && substr($s, $i, 1) ne substr($s, $k, 1) && substr($s, $j, 1) ne substr($s, $k, 1);
  }
}
say ($r * $g * $b - $ans);