#define _GLIBCXX_DEBUG
#define _USE_MATH_DEFINES

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

#define rep0(N) for (int COUNTER = 0; COUNTER < (int)(N); COUNTER++)
#define rep(i, N) for (int i = 0; i < (int)(N); i++)
#define rep1(i, N) for (int i = 0; i < (int)(N); i++)
#define rep2(i, START, GOAL) for (int i = (int)(START); i < (int)(GOAL); i++)
#define rep3(i, START, GOAL) for (int i = (int)(START); i > (int)(GOAL); i--)

#define all(CONTAINER) CONTAINER.begin(), CONTAINER.end()
#define rall(CONTAINER) CONTAINER.rbegin(), CONTAINER.rend()
#define from1(CONTAINER) CONTAINER.begin() + 1, CONTAINER.end()
#define rfrom1(CONTAINER) CONTAINER.rbegin(), CONTAINER.rend() - 1

#define pout(X) cout << X << " "
#define print(X) cout << X << "\n"
#define output(X) cout << X << "\n"

#define dbe(X) cerr << X << " "
#define dbel(X) cerr << X << "\n"
#define dberr(X) cerr << X << " "
#define dberrl(X) cerr << X << "\n"

#define db(X) cerr << #X << ":" << (X) << " "
#define dbl(X) cerr << #X << ":" << (X) << "\n"

#define db2(X, Y) cerr << #X << ":" << (X) << ", " << #Y << ":" << (Y) << " "
#define db2l(X, Y) cerr << #X << ":" << (X) << ", " << #Y << ":" << (Y) << "\n"
#define dbl2(X, Y) cerr << #X << ":" << (X) << "\n" << #Y << ":" << (Y) << "\n"
#define db3(X, Y, Z) cerr << #X << ":" << (X) << ", " << #Y << ":" << (Y) << " " << #Z << ":" << (Z) << " "
#define db3l(X, Y, Z) cerr << #X << ":" << (X) << ", " << #Y << ":" << (Y) << ", " << #Z << ":" << (Z) << "\n"
#define dbl3(X, Y, Z) cerr << #X << ":" << (X) << "\n" << #Y << ":" << (Y) << "\n" << #Z << ":" << (Z) << "\n"

#define dbp(PAIR) cerr << #PAIR << ":(" << PAIR.first << ", " << PAIR.second << ") "
#define dbpl(PAIR) cerr << #PAIR << ":(" << PAIR.first << ", " << PAIR.second << ")\n"
#define dbt3(TUPLE3) cerr << #TUPLE3 << ":(" << get<0>(TUPLE3) << ", " << get<1>(TUPLE3) << ", " << get<2>(TUPLE3) << ") "
#define dbt3l(TUPLE3) cerr << #TUPLE3 << ":(" << get<0>(TUPLE3) << ", " << get<1>(TUPLE3) << ", " << get<2>(TUPLE3) << ")\n"
#define dbt4(TUPLE4) cerr << #TUPLE4 << ":(" << get<0>(TUPLE4) << ", " << get<1>(TUPLE4) << ", " << get<2>(TUPLE4) << ", " << get<3>(TUPLE4) << ") "
#define dbt4l(TUPLE4) cerr << #TUPLE4 << ":(" << get<0>(TUPLE4) << ", " << get<1>(TUPLE4) << ", " << get<2>(TUPLE4) << ", " << get<3>(TUPLE4) << ")\n"

#define dbv(VEC) cerr << #VEC << ":{ "; for (auto ELEM : VEC) cerr << ELEM << ", "; cerr << "}\n"
#define dbvp(VP) cerr << #VP << ":{ "; for (auto PAIR : VP) cerr << "(" << PAIR.first << ", " << PAIR.second << "), "; cerr << "}\n"

#define dbvv(VV) cerr << #VV << ":{\n"; for (auto VEC : VV) { cerr << "{ "; for (auto ELEM : VEC) cerr << ELEM << ", "; cerr << "},\n"; } cerr << "}\n"
#define dbvvp(VVP) cerr << #VVP <<":{\n"; for (auto VP : VVP) { cerr << "{ "; for (auto PAIR : VP) { cerr << "(" << PAIR.first << ", " << PAIR.second << "), "; } cerr << "},\n"; } cerr << "}\n";

#define dbs(SET) cerr << #SET << "{ "; for (auto ELEM : SET) cerr << ELEM << ", "; cerr << "}\n";
#define dbsp(SP) cerr << #SP << "{ "; for (auto PAIR : SP) cerr << "(" << PAIR.first << ", " << PAIR.second << "), "; "}\n";

#define dbm(MAP) cerr << #MAP << ":{ "; for (auto PAIR : MAP) cerr << "(" << PAIR.first << ", " << PAIR.second << "), "; cerr << "}\n"

#define YN(f) cout << (f ? "YES" : "NO") << "\n"
#define Yn(f) cout << (f ? "Yes" : "No") << "\n"
#define yn(f) cout << (f ? "yes" : "no") << "\n"

using ll = long long;

using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pdd = pair<double, double>;

using ti3 = tuple<int, int, int>;
using tl3 = tuple<ll, ll, ll>;
using td3 = tuple<double, double, double>;

using ti4 = tuple<int, int, int, int>;
using tl4 = tuple<ll, ll, ll, ll>;
using td4 = tuple<double, double, double, double>;

using vi = vector<int>;
using vl = vector<ll>;
using vd = vector<double>;
using vs = vector<string>;
using vb = vector<bool>;

using vvi = vector<vi>;
using vvl = vector<vl>;
using vvd = vector<vd>;
using vvb = vector<vb>;

using vpii = vector<pii>;
using vpll = vector<pll>;
using vpdd = vector<pdd>;

using mii = map<int, int>;
using mll = map<ll, ll>;

using si = set<int>;
using sl = set<ll>;
using ss = set<string>;

using spii = set<pii>;
using spll = set<pll>;
using spdd = set<pdd>;
// db

int main() {
	int N; cin >> N;
	string S; cin >> S;
	ll nr = 0, ng = 0, nb = 0;
	for (char c : S) {
		if (c == 'R') nr++;
		else if (c == 'G') ng++;
		else if(c == 'B') nb++;
	}
	ll ans = nr * ng * nb;
	// dbl(ans);
	for (int d = 1; 2 * d < N; d++) {
		for (int i = 0; i + 2 * d < N; i++) {
			// db3(i, i + d, i + 2 * d);
			// db3l(S[i], S[i + d], S[i + 2 * d]);
			if (S[i] != S[i + d] and S[i] != S[i + 2 * d] and S[i + d] != S[i + 2 * d]) ans--;
		}
	}
	print(ans);
}

Measured memory_bytes: 3632