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 C (GCC 9.2.1) (judge verdict: Wrong Answer)

#include <stdio.h>

int main(){
  int n;  scanf("%d", &n);
  char s[4010];  scanf("%s", s);
  int n_r = 0, n_g = 0, n_b = 0;
  for(int i=0; i<n; i++)
    if(s[i] == 'R')  n_r++;
    else if(s[i] == 'G')  n_g++;
    else if(s[i] == 'B')  n_b++;
  
  int ans = n_r * n_g * n_b;

  for(int i=0; i<n-2; i++){
    for(int j=i+1; j<n-1; j++){
      if(s[i] == s[j])  continue;
      int k = j*2-i;
      if(k >= n)  continue;
      if(s[k] != s[i] && s[k] != s[j])
	ans--;
    }
  }

  printf("%d\n", ans);

  return 0;
}