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: Accepted)

#include <stdio.h>

int main(){
  long int num,sum;
  scanf("%ld",&num);
  char s[4000]={};
  scanf("%s",s,num);
  long int count_r=0,count_g=0,count_b=0;
  for(int a=0;a<=num-1;a++){
    if(s[a]=='R')count_r++;
    else if(s[a]=='G')count_g++;
    else count_b++;
  };
  sum=count_r*count_g*count_b;
  long int i,j,k;
  for(j=0;j<=num-2;j++){
    for(i=j+1;2*i-j<=num-1;i++){
      k=2*i-j;
      if(s[j]!=s[i]&&s[k]!=s[i]&&s[k]!=s[j])
        sum--;
    };
  };
   printf("%ld",sum);
};