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

#define FOR(i, a, b) for(int i = (a); i < (b); ++i)
#define REP(i, n) for(int i = 0; i < (n); ++i)
#define REPr(i, n) for(int i = (n)-1; i >= 0; --i)
#define FORq(i, m, n) for(int i = (m); i <= (n); ++i)
#define FORqr(i, m, n) for(int i = (n); i >= (m); --i)
#define PB push_back
#define MP make_pair
#define fst first
#define snd second
#define SIN(x, S) (S.count(x) != 0)
#define M0(x) memset(x, 0, sizeof(x))
#define FILL(x, y) memset(x, y, sizeof(x))
#define MM(x) memset(x, -1, sizeof(x))
#define ALL(x) (x).begin(), (x).end()
#define DB(x) cerr << #x << " = " << x << endl
#define DB2(x, y)                                                              \
    cerr << "(" << #x << ", " << #y << ") = (" << x << ", " << y << ")\n";
#define DEBUG                                                                  \
    int x12345;                                                                \
    cin >> x12345;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<long long> VL;
typedef long long ll;
typedef long long integer;
///////////////////////////////////////////////
const ll MOD = 1e9 + 7;
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }

template <class T> inline bool chmin(T &a, T b) {
    if(a > b) {
        a = b;
        return true;
    }
    return false;
}

template <class T> inline bool chmax(T &a, T b) {
    if(a < b) {
        a = b;
        return true;
    }
    return false;
}

template <int MOD> struct Fp {
    long long val;

    constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
        if(val < 0)
            v += MOD;
    }

    constexpr int getmod() { return MOD; }

    constexpr Fp operator-() const noexcept { return val ? MOD - val : 0; }

    constexpr Fp operator+(const Fp &r) const noexcept {
        return Fp(*this) += r;
    }

    constexpr Fp operator-(const Fp &r) const noexcept {
        return Fp(*this) -= r;
    }

    constexpr Fp operator*(const Fp &r) const noexcept {
        return Fp(*this) *= r;
    }

    constexpr Fp operator/(const Fp &r) const noexcept {
        return Fp(*this) /= r;
    }

    constexpr Fp &operator+=(const Fp &r) noexcept {
        val += r.val;
        if(val >= MOD)
            val = val % MOD;
        return *this;
    }

    constexpr Fp &operator-=(const Fp &r) noexcept {
        val -= r.val;
        if(val < 0)
            val += MOD * ((-val + MOD - 1) / MOD);
        val = val % MOD;
        return *this;
    }

    constexpr Fp &operator*=(const Fp &r) noexcept {
        val = val * r.val % MOD;
        return *this;
    }

    constexpr Fp &operator/=(const Fp &r) noexcept {
        long long a = r.val, b = MOD, u = 1, v = 0;
        while(b) {
            long long t = a / b;
            a -= t * b;
            swap(a, b);
            u -= t * v;
            swap(u, v);
        }
        val = val * u % MOD;
        if(val < 0)
            val += MOD;
        return *this;
    }

    constexpr bool operator==(const Fp &r) const noexcept {
        return this->val == r.val;
    }

    constexpr bool operator!=(const Fp &r) const noexcept {
        return this->val != r.val;
    }

    friend constexpr ostream &operator<<(ostream &os,
                                         const Fp<MOD> &x) noexcept {
        return os << x.val;
    }

    friend constexpr istream &operator>>(istream &is, Fp<MOD> &x) noexcept {
        return is >> x.val;
    }

    friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, long long n) noexcept {
        if(n == 0)
            return 1;
        auto t = modpow(a, n / 2);
        t = t * t;
        if(n & 1)
            t = t * a;
        return t;
    }
};

using mint = Fp<MOD>;

///////////////////////////////////////////////
/// 🍈( '-' 🍈 |AC|
ll gcd3(ll a, ll b, ll c){
    return gcd(a,gcd(b,c));
}

int rest(int a, int b){
    if (a == b) return -1;
    if ((a != 0) and (b != 0)) return 0;
    if ((a != 1) and (b != 1)) return 1;
    if ((a != 2) and (b != 2)) return 2;
}

ll lsum[4002][3];
int ctoi (const char c){
  if ('0' <= c && c <= '9') return (c - '0');
  return -1;
}

int cl(const char c){
    if (c == 'R') return 0;
    if (c == 'G') return 1;
    if (c == 'B') return 2;
}

int main() {
    ll N; cin >> N;
    string S; cin >> S;
    ll ans = 0;
    S = 'x' + S;
    for(int i = 1; i <= N; i++){
        char c = S[i];
        if (c == 'R') lsum[i][0]++;
        if (c == 'G') lsum[i][1]++;
        if (c == 'B') lsum[i][2]++;
        lsum[i][0] += lsum[i-1][0];
        lsum[i][1] += lsum[i-1][1];
        lsum[i][2] += lsum[i-1][2];
    }

    for (int j = 2; j <= N - 1; j++){
        for(int i = 1; i < j; i++){
            int nowc = cl(S[j]);
            int nowl = cl(S[i]);
            int restc = rest(nowc,nowl);
            if (restc == -1) continue;
            ll restnum = lsum[N][restc] - lsum[j][restc];
            if ((cl(S[(2*j-i)]) == restc) and (2*j-i <= N)){
                restnum--;
            }
            ans += restnum;
        }
    }

    cout << ans << endl;
}

Measured memory_bytes: 3736