//#include <bits/stdc++.h>
#include "bits/stdc++.h"
#define rep(i, n) for (int i = 0; i < (n); ++i)
using namespace std;
typedef long long ll;
typedef pair<int, int> P;
typedef vector<int> VI;
typedef vector<VI> VVI;//例: VVI dp(10, vector<int>(10, INF);
typedef priority_queue<int, vector<int>, less<int> > QUE_int;
const int INF = 2 * pow(10, 9) + 1;//+1しないとREになるかも(out of rangeになるんかな?? )

vector<int> dx = { -1, 0, 1, 0, -1, -1, 1, 1 };
vector<int> dy = { 0, -1, 0, 1, -1, 1, -1, 1 };

ll max(ll a, ll b) { return (a > b) ? a : b; }
ll min(ll a, ll b) { return (a < b) ? a : b; }

////////////////////////////////////////////////////////////////

// auto mod int
// https://youtu.be/L8grWxBlIZ4?t=9858
// https://youtu.be/ERZuLAxZffQ?t=4807 : optimize
// https://youtu.be/8uowVvQ_-Mo?t=1329 : division
const int mod = pow(10, 9) + 7;
struct mint {
	ll x; // typedef long long ll;
	mint(ll x = 0) :x((x%mod + mod) % mod) {}
	mint operator-() const { return mint(-x); }
	mint& operator+=(const mint a) {
		if ((x += a.x) >= mod) x -= mod;
		return *this;
	}
	mint& operator-=(const mint a) {
		if ((x += mod - a.x) >= mod) x -= mod;
		return *this;
	}
	mint& operator*=(const mint a) {
		(x *= a.x) %= mod;
		return *this;
	}
	mint operator+(const mint a) const {
		mint res(*this);
		return res += a;
	}
	mint operator-(const mint a) const {
		mint res(*this);
		return res -= a;
	}
	mint operator*(const mint a) const {
		mint res(*this);
		return res *= a;
	}
	mint pow(ll t) const {
		if (!t) return 1;
		mint a = pow(t >> 1);
		a *= a;
		if (t & 1) a *= *this;
		return a;
	}

	// for prime mod
	mint inv() const {
		return pow(mod - 2);
	}
	mint& operator/=(const mint a) {
		return (*this) *= a.inv();
	}
	mint operator/(const mint a) const {
		mint res(*this);
		return res /= a;
	}
};
// combination mod prime
// https://www.youtube.com/watch?v=8uowVvQ_-Mo&feature=youtu.be&t=1619
struct combination {
	vector<mint> fact, ifact;
	combination(int n) :fact(n + 1), ifact(n + 1) {
		assert(n < mod);
		fact[0] = 1;
		for (int i = 1; i <= n; ++i) fact[i] = fact[i - 1] * i;
		ifact[n] = fact[n].inv();
		for (int i = n; i >= 1; --i) ifact[i - 1] = ifact[i] * i;
	}
	mint operator()(int n, int k) {
		if (k < 0 || k > n) return 0;
		return fact[n] * ifact[k] * ifact[n - k];
	}
};

template <typename TYPE>
void print_vec(const vector<TYPE>& v){
	for(int i=0; i<v.size(); i++){
		cout << v[i] << " ";
	}
	cout << endl;
}

ll gcd(ll a, ll b){//greatest common divider 最大公約数を求める
  if(b==0) return a;
  return gcd(b, a%b);
}

////////////////////////////////////////////////////////////////

int main(void) {
	int N;
	cin >> N;
	string S;
	cin >> S;

	vector<int> s_int(N, -1);
	int r_num = 0, g_num=0, b_num=0;
	for(int i=0; i<N; i++){
		if(S[i]=='R'){
			s_int[i] = 0;
			r_num++;
		}
		else if(S[i]=='G'){
			s_int[i] = 1;
			g_num++;
		}
		else if(S[i]=='B'){
			s_int[i] = 2;
			b_num++;
		}
	}

	//ll rule1 = r_num * g_num * b_num;//条件１を満たす個数
	ll rule1 = 1;
	rule1 *= r_num;
	rule1 *= g_num;
	rule1 *= b_num;
	ll rule1_not2 = 0;//条件1を満たすが条件2を満たさない個数
	ll ans = 0; // ans = rule1 - rule1_not2;
	for(int i=0; i<N; i++){
		for(int j=i; j<N; j++){
			int k = 2*j - i;
			if(k < 0 || N <= k) continue;
			if(S[i] != S[j] && S[i] != S[k] && S[j] != S[k]){
				//cout << " i:" << i << " j: " << j << " k: " << k << endl;
				rule1_not2++;
			}
		}
	}
	
	//cout << " rule1: " << rule1 << " rule1_not2: " << rule1_not2 << endl;
	ans = rule1 - rule1_not2;
	cout << ans << endl;
}


Measured memory_bytes: 3644