#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
/*
*/

ll gcd(ll a,ll b){
  if(b == 0) return a;
  return gcd(b, a % b);
}

ll lcm(ll a, ll b){
  return a/gcd(a,b) * b;
}
ll func2(ll n){
  ll res = 0;
  while(n%2 == 0){
    n /= 2;
    res++;
  }
  return res;
}

ll zyou(ll a,ll b){//a＾b
  ll res = 1;
  for(int i = 1;i<=b;i++){
    res *= a;
  }
  return res;
}
struct Edge{
  int to;
  int weight;
  Edge(int t, int w) : to(t),weight(w){ }
};

typedef pair<int,int> P;
using Graph = vector<vector<int>>;

ll calc(ll a, ll b, ll p){//aのb乗をpで割った余り
  if(b == 0) return 1;

  else if(b%2 == 0){
    ll d = calc(a,b/2,p);
    return (d*d)%p;
  }
  else if(b%2 == 1){
    return (a * calc(a,b-1,p))%p;
  }
}


bool is_prime(int n){//nが素数であるかどうか判定 iで割り切れると素数でない
    if(n == 1) return false;//1は例外
    for(int i = 2;i*i <= n;i++){
        if(n%i == 0) return false;
    }
    
    return true;
}

bool palin(string S){//回文判定
  int n = S.size();
  bool flag = true;
  if(n%2 == 0){
    for(int i = 0;i<=n/2-1;i++){
      if(S[i] != S[n-i-1]){
        flag = false;
        break;
      }
    }
  }
  else if(n%2 == 1){
    for(int i = 0;i<=n/2-1;i++){
      if(S[i] != S[n-i-1]){
        flag = false;
        break;
      }
    }
  }
  return flag;
}
int main(){
    int N;
    string S;
    cin >> N;
    cin >> S;
    ll ans2 = 0;
    ll r = 0;
    ll b = 0;
    ll g = 0;
    for (int i = 0; i < N; i++) {
      if (S[i] == 'R') r++;
      if (S[i] == 'G') g++;
      if (S[i] == 'B') b++;
    }
    ll ans1 = r * g * b;

    for (int i = 0; i < N; i++) {
        for (int j = i+1; j < N; j++) {
          if (S[i] != S[j]) {
            int k = 2 * j - i;
            if(k >= N) continue;
            if (S[i] != S[k] && S[j] != S[k]) ans2++;
          }
        }
    }
    cout << ans1 - ans2 << endl;
}