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 Julia (1.4.0) (judge verdict: Accepted)

function countRGB(s)
    numR = numG = numB = 0
    for c in s
        if c == 'R'
            numR += 1
        elseif c == 'G'
            numG += 1
        elseif c == 'B'
            numB += 1
        end
    end
    (numR, numG, numB)
end

function numequaldistance(s)
    ret = 0
    for i = 1:length(s)-2
        for d = 1:(length(s)-i)÷2
            j = i+d
            k = j+d
            if s[i] != s[j] && s[j] != s[k] && s[k] != s[i]
                ret += 1
            end
        end
    end
    ret
end

function solve()
    readline()
    s = readline()
    numR, numG, numB = countRGB(s)
    ned = numequaldistance(s)
    numR*numG*numB - ned
end

println(solve())