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

typedef long long ll;
typedef pair<int, int> pi;
typedef pair<ll, ll> pll;

typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<bool> vb;
typedef vector<pi> vpi;
typedef vector<vi> vvi;
typedef vector<vpi> vvpi;
typedef vector<vll> vvll;
typedef vector<vb> vvb;
typedef vector<vvi> vvvi;

#define all(x) x.begin(), x.end()

const int MOD = 1000000009;
const int INF = 2e9;

//TO_STRING
template <typename A, typename B>
string to_string(pair<A, B> p);
 
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);
 
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);
 
string to_string(const string& s) {
  return '"' + s + '"';
}

string to_string(char ch){
	string s;
	s += ch;
	return s;
}

string to_string(const char* s) {
  return to_string((string) s);
}
 
string to_string(bool b) {
  return (b ? "true" : "false");
}
 
string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
 
template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}
 
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
 
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
 
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + 
  	to_string(get<2>(p)) + ")";
}
 
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + 
  	to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}

//DEBUG 
void debug_out() { cerr << endl; }
 
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL //compile with -DLOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

#define pb push_back
#define fi first
#define se second  

void solve() {
  map<char, int> id = {{'R', 0}, {'G', 1}, {'B', 2}};
  int n;
  string s;
  cin >> n >> s;
  s.insert(s.begin(), 1, '@');
  vvi cumsum(3, vi(n + 1));
  for (int i = 1; i <= n; i++) {
    for (int col = 0; col < 3; col++) {
      cumsum[col][i] = cumsum[col][i - 1];
    }
    cumsum[id[s[i]]][i]++;
  }
  debug(cumsum);
  ll sum = 0;
  for (int i = n; i >= 1; i--) {
    for (int j = i - 1; j >= 1; j--) {
      if (s[i] == s[j]) continue;
      int curid = 0;
      for (curid = 0; curid < 3; curid++) {
        if (curid != id[s[i]] && curid != id[s[j]]) {
          break;
        }
      }
      sum += cumsum[curid][j - 1];
      int checkid = 2 * j - i;
      if (checkid >= 1 && id[s[checkid]] == curid) {
        sum--;
      }
    }
  }
  cout << sum << endl;
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);cout.tie(0);	
  int t = 1;
  // cin >> t;
  while (t--) {
    solve();
  }
} 

Measured memory_bytes: 3704