// template version 1.15
using namespace std;
#include <bits/stdc++.h>

// varibable settings
#define int long long
const int INF=1e18;

// define basic macro {{{
#define _overload3(_1,_2,_3,name,...) name
#define _rep(i,n) repi(i,0,n)
#define repi(i,a,b) for(int i=(int)(a);i<(int)(b);++i)
#define rep(...) _overload3(__VA_ARGS__,repi,_rep,)(__VA_ARGS__)
#define _rrep(i,n) rrepi(i,0,n)
#define rrepi(i,a,b) for(int i=(int)((b)-1);i>=(int)(a);--i)
#define rrep(...) _overload3(__VA_ARGS__,rrepi,_rrep,)(__VA_ARGS__)
#define each(i,a) for (auto&& i : a)
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define sz(x) ((int)(x).size())
#define pb(a) push_back(a)
#define mp(a, b) make_pair(a, b)
#define mt(a, b, c) make_tuple(a, b, c)
#define ub upper_bound
#define lb lower_bound
#define posl(A, x) (lower_bound(all(A), x)-A.begin())
#define posu(A, x) (upper_bound(all(A),x)-A.begin())
template<class T> inline void chmax(T &a, const T &b) { if((a) < (b)) (a) = (b); }
template<class T> inline void chmin(T &a, const T &b) { if((a) > (b)) (a) = (b); }

#define divceil(a,b) ((a)+(b)-1)/(b)
#define is_in(x, a, b) ((a)<=(x) && (x)<(b))
#define uni(x) sort(all(x));x.erase(unique(all(x)),x.end())
#define slice(l, r) substr(l, r-l)

typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef long double ld;
typedef pair<int,int> pii;
typedef tuple<int,int,int> iii;

template<typename T> using PQ = priority_queue<T, vector<T>, greater<T>>;
struct Fast { Fast(){ std::cin.tie(0); ios::sync_with_stdio(false); } } fast;

#if defined(PCM) || defined(LOCAL)
    #include "lib/dump.hpp"
#else
    #define dump(...) 42
    #define dump_1d(...) 42
    #define dump_2d(...) 42
    #define cerrendl 42
#endif
//}}}

int n;
string s;

// acumulative sum vector {{{
template<typename T>
struct asvector {
    vector<T> cum; //accumulative sum
    asvector(vector<T> a): cum(a){  // {{{  // constructor
        rep(i, 1, sz(a))  cum[i]+=cum[i-1];
    } // }}}

    T rsum(int l, int r) {  // return sum of [l, r) of data.  {{{
        l = max(0LL, l);
        r = min(r, sz(cum));
        if (l<r){
            dump(l, r);
            return cum[r-1] - (l-1>=0 ? cum[l-1] : 0);
        }
        else{
            return 0;
        }
    } // }}}

    T& operator[](int i) { // {{{
        return cum[i]; 
    } // }}}

    friend ostream& operator<<(ostream &os, asvector<T>& as){//{{{
        os << as.cum; return os;
    } //}}}
};
//}}}

int count(char L, char M, char R){
    cerrendl;
    int res = 0;
    vi l,m,r;

    vi exist(4001);
    vi lv(4001);
    vi mv(4001);
    vi rv(4001);
    // asvector<int> l;
    rep(i, n){
        if (s[i]==L) l.pb(i), lv[i]=1;
        if (s[i]==M) m.pb(i), mv[i]=1;
        if (s[i]==R) r.pb(i), rv[i]=1;
    }
    asvector<int> alv(lv);
    asvector<int> amv(mv);
    asvector<int> arv(rv);

    each(x, m){
        int lc = alv.rsum(0, x);
        int rc = arv.rsum(x+1, n);
        dump(x, lc, rc);
        res += lc * rc;
    }

    dump(res);
    each(li, l){
        each(ri, r){
            dump(ri, li);
            if (ri-li>0 && (ri-li)%2==0 && mv[(ri+li)/2]){
                dump("hey");
                res--;
            }
        }
    }
    return res;
}

signed main() {
    cin>>n;
    cin>>s;
    int ans = 0;
    ans += count('R', 'G', 'B');
    ans += count('R', 'B', 'G');
    ans += count('G', 'R', 'B');
    ans += count('G', 'B', 'R');
    ans += count('B', 'R', 'G');
    ans += count('B', 'G', 'R');
    cout << ans << endl;

    return 0;
}


Measured memory_bytes: 3884