/* Aa^~ kokoro ga pyonpyon suru n jaa^~
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*/

#include <bits/stdc++.h>
using namespace std;

using VI = vector<int>;
using VVI = vector<VI>;
using PII = pair<int, int>;
using LL = long long;
using VL = vector<LL>;
using VVL = vector<VL>;
using PLL = pair<LL, LL>;
using VS = vector<string>;

#define ALL(a)  begin((a)),end((a))
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define MT make_tuple
#define SZ(a) int((a).size())
#define SORT(c) sort(ALL((c)))
#define RSORT(c) sort(RALL((c)))
#define UNIQ(c) (c).erase(unique(ALL((c))), end((c)))

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

#define FF first
#define SS second

template<class S, class T>
istream& operator>>(istream& is, pair<S,T>& p){
  return is >> p.FF >> p.SS;
}
template<class T>
istream& operator>>(istream& is, vector<T>& xs){
  for(auto& x: xs)
	is >> x;
  return is;
}
template<class S, class T>
ostream& operator<<(ostream& os, const pair<S,T>& p){
  return os << p.FF << " " << p.SS;
}
template<class T>
ostream& operator<<(ostream& os, const vector<T>& xs){
  for(unsigned int i=0;i<xs.size();++i)
	os << (i?" ":"") << xs[i];
  return os;
}
template<class T>
void maxi(T& x, T y){
  if(x < y) x = y;
}
template<class T>
void mini(T& x, T y){
  if(x > y) x = y;
}
void debug(istringstream&){}
template <char sep=',', class Head, class... Tail>
void debug(istringstream& iss, Head&& head, Tail&&... tail)
{
  string name;
  getline(iss, name, ',');
  cout << sep << name << "=" << head;
  debug(iss, forward<Tail>(tail)...);
}

#ifdef PYONPOI
#define DEBUG(...) \
  do{ \
    istringstream ss(#__VA_ARGS__); \
    debug<' '>(ss, __VA_ARGS__); \
    cout<<endl; \
  }while(0)
#else
#define DEBUG
#endif
void init_io(int argc, char* argv[]){
  cin.tie(0);
  ios_base::sync_with_stdio(false);
#ifdef PYONPOI
  if(argc > 1) {
    ifstream *ifs = new ifstream();
    ifs->open(argv[1]);
    cin.rdbuf(ifs->rdbuf());
  }
#endif
}


const double EPS = 1e-10;
const double PI  = acos(-1.0);
const LL MOD = 1e9+7;

int main(int argc, char* argv[]){
  init_io(argc, argv);

  int N;
  cin >> N;
  string S;
  cin >> S;

  string rgb = "RGB";
  auto toi = [](char c){
    if(c == 'R') return 0;
    if(c == 'G') return 1;
    if(c == 'B') return 2;
    return -1;
  };
  VVL cnt(3, VL(N+1, 0));

  for(int i=N-1;i>=0;--i){
    REP(c,3) cnt[c][i] = cnt[c][i+1];
    auto c = S[i];
    REP(ix,3) if(c == rgb[ix]) cnt[ix][i]++;
  }

  LL ans = 0;
  REP(j,N){
    REP(i,j) {
      if(S[i] == S[j]) continue;
      set<int> s = {toi(S[i]), toi(S[j])};
      int ix;
      REP(ii,3){ix = ii; if(!s.count(ii)) break; }
      ans += cnt[ix][j+1];
      
      int bk = 2 * j - i;
      if(0 <= bk && bk < N && S[bk] == rgb[ix]) --ans;
    }
  }
  cout << ans << endl;

  return 0;
}


Measured memory_bytes: 3752