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 RGBTriplets()
    N = parse(Int, readline())
    S = readline()
    r, g, b = 0, 0, 0
    for s in S
        if s =='R'
            r+=1
        elseif s == 'G'
            g+=1
        else
            b+=1
        end
    end
    counter = 0
    for i in 1:N-1
        for j in i+1:N
            k = 2j - i
            k > N && break
            (S[i] ≠ S[j]) && (S[j] ≠ S[k]) && (S[k] ≠ S[i]) && (counter += 1)
        end
    end
    return println(r*g*b - counter)
end
RGBTriplets()