/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author Rishabhdeep Singh
 */

// doot diddly donger cuckerino Hahahahahah
#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")
#pragma GCC target("popcnt")
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace __gnu_pbds;
using namespace __gnu_cxx;
using namespace std;

// @formatter:off
#define PB              push_back
#define EB              emplace_back
#define F               first
#define S               second
#define all(x)          (x).begin(), (x).end()
#define rall(x)         rbegin(x), rend(x)
#define sz(x)           (long long) (x).size()
#define NL              << '\n'
#define IOS             ios::sync_with_stdio(false);cin.tie(NULL); cout << fixed << setprecision(12);
#define IN_REP          int tt; cin >> tt; while(tt--)
#define sim             template<typename T
#define ris             return *this
#define dor             > debug & operator <<
#define eni(x)          sim > typename enable_if<sizeof dud<T>(0) x 1, debug&>::type operator<<(T i)
sim> struct rge { T b, e; };
sim> rge<T> range(T i, T j) { return rge<T>{i, j}; }
sim> auto dud(T *x) -> decltype(cerr << *x, 0);
sim> char dud(...);
struct debug {
#ifdef LOCAL
  ~debug() { cerr << endl; }
  eni(!=) { cerr << boolalpha << i; ris; }
  eni(==) { ris << range(all(i)); }
  sim, typename U dor(pair<U, T> d) { ris << "(" << d.first << ", " << d.second << ")"; }
  sim dor(rge<T> d) { *this << "["; for (auto it = d.b; it != d.e; ++it) *this << ", " + 2 * (it == d.b) << *it; ris << "]"; }
#else
  sim dor(const T&) { ris; }
#endif
};
#define pr(...) " [" << #__VA_ARGS__ ": " << (__VA_ARGS__) << "] "

using ll = long long;
using ull = unsigned long long;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vi = vector<int>;
using vb = vector<bool>;

using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vpii = vector<pii>;
using vpll = vector<pll>;
template <typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

const ld PI = acos(-1);

template<typename T> T gcd(T x, T y) { return (y == 0) ? x : gcd(y, x % y); }
template<typename T> void mini(T& x, T y) { x = min(x, y); }
template<typename T> void maxi(T& x, T y) { x = max(x, y); }
template<typename T, typename U> string to_string(pair<T, U> p) { return to_string(p.F) + " " + to_string(p.S); }
mt19937 rnd(chrono::high_resolution_clock::now().time_since_epoch().count());
// @formatter:on


class DRGBTriplets {
public:

  ll nC3(ll n) { return n * (n - 1) * (n - 2) / 6; }
  ll nC2(ll n) { return n * (n - 1) / 2; }

  void solve(std::istream &cin, std::ostream &cout) {
    int n;
    cin >> n;
    string s;
    cin >> s;

    ll nR = count_if(all(s), [&](char c) { return c == 'R'; });
    ll nG = count_if(all(s), [&](char c) { return c == 'G'; });
    ll nB = count_if(all(s), [&](char c) { return c == 'B'; });

    ll res = nC3(n) - nC3(nR) - nC3(nG) - nC3(nB);
    res -= nC2(nR) * (nG + nB);
    res -= nC2(nG) * (nR + nB);
    res -= nC2(nB) * (nG + nR);

    for (int i = 0; i < n; ++i) {
      for (int j = 1; j < n; ++j) {
        int ll = i - j, rr = j + i;
        if (ll >= 0 and ll < n and rr >= 0 and rr < n and s[ll] != s[i] and s[rr] != s[i] and s[ll] != s[rr]) --res;
      }
    }

    cout << res << '\n';
  }
};

int main() {
    IOS;
	DRGBTriplets solver;
	std::istream& in(std::cin);
	std::ostream& out(std::cout);
	solver.solve(in, out);
	return 0;
}


Measured memory_bytes: 3644