// header {{{
// header {{{
using namespace std;
#include <bits/stdc++.h>
 
#define CPP_STR(x) CPP_STR_I(x)
#define CPP_CAT(x,y) CPP_CAT_I(x,y)
#define CPP_STR_I(args...) #args
#define CPP_CAT_I(x,y) x ## y
 
#define ASSERT(expr...) assert((expr))
 
using i8  = int8_t;
using u8  = uint8_t;
using i16 = int16_t;
using u16 = uint16_t;
using i32 = int32_t;
using u32 = uint32_t;
using i64 = int64_t;
using u64 = uint64_t;
 
using f32 = float;
using f64 = double;
// }}}
 
constexpr i64 INF = 1'010'000'000'000'000'017LL;
 
constexpr i64 MOD = 1'000'000'007LL;
 
constexpr f64 EPS = 1e-12;
 
constexpr f64 PI = 3.14159265358979323846;

#define M5 100007
#define M9 1000000000

 
// util {{{
#define FOR(i, start, end) for(i64 i = (start), CPP_CAT(i,xxxx_end)=(end); i < CPP_CAT(i,xxxx_end); ++i)
#define REP(i, n) FOR(i, 0, n)
 
#define ALL(f,c,...) (([&](decltype((c)) cccc) { return (f)(std::begin(cccc), std::end(cccc), ## __VA_ARGS__); })(c))
#define ll long long int
#define VI vector <int>
template<typename C>
i64 SIZE(const C& c) { return static_cast<i64>(c.size()); }
 
template<typename T, size_t N>
i64 SIZE(const T (&)[N]) { return static_cast<i64>(N); }
 
template<typename T, typename U, typename Comp=less<>>
bool chmax(T& xmax, const U& x, Comp comp={}) {
    if(comp(xmax, x)) {
        xmax = x;
        return true;
    }
    return false;
}
 
template<typename T, typename U, typename Comp=less<>>
bool chmin(T& xmin, const U& x, Comp comp={}) {
    if(comp(x, xmin)) {
        xmin = x;
        return true;
    }
    return false;
}
// }}}
 
// init {{{
struct ProconInit {
    static constexpr int IOS_PREC = 15;
    static constexpr bool AUTOFLUSH = false;
 
    ProconInit() {
        cin.tie(nullptr);
        ios::sync_with_stdio(false);
        cout << fixed << setprecision(IOS_PREC);
        if(AUTOFLUSH)
            cout << unitbuf;
    }
} PROCON_INIT;
// }}}
 
//--------------------------------------------------------------------
 
bool isOK(int mid, double key){
 
    if((double)mid*mid > key) {
             return true;
     } else {
              return false;
     }
}

 int binary_search(double key) {
    int left = -1; //「index = 0」が条件を満たすこともあるので、初期値は -1
    int right = 2000000000;
    /* どんな二分探索でもここの書き方を変えずにできる！ */
    while (right - left > 1) {
        int mid = left + (right - left) / 2;

        if (isOK(mid, key)) right = mid;
        else left = mid;
    }

    /* left は条件を満たさない最大の値、right は条件を満たす最小の値になっている */
    return right;
}


bool isKaibun(string st){
    bool ans = true;
    int n = st.length();
    REP(i,n/2){
        if(st[i] != st[n-i-1]){
            return false;
        }
    }
    return true;
}

int toInt(char a){
    return a-'0';
}

int gcd(int a, int b)
{
   if (a%b == 0)
   {
       return(b);
   }
   else
   {
       return(gcd(b, a%b));
   }
}

int lcm(int a, int b)
{
   return a * b / gcd(a, b);
}

void solve() {
    int N;
    string S;
    cin >> N >> S;
    VI R,G,B;
    G.resize(N+1);
    B.resize(N+1);
    R.resize(N+1);
    G[N] = 0;
    R[N] = 0;
    B[N] = 0;
    REP(i,N-1){
        int a = N-i;
        R[a-2] = R[a-1];
        B[a-2] = B[a-1];
        G[a-2] = G[a-1];
        switch(S[a-1]){
            case 'R':
                R[a-2]++;
                break;
            case 'G':
                G[a-2]++;
                break;
            case 'B':
                B[a-2]++;
                break;
        }
    }
/*
    REP(i,S.length()){
        cout << R[i]<<" " << G[i] <<" "<< B[i] << endl;
    }
*/
    ll ans = 0;
    REP(i,N-1){
        FOR(j,i+1,N){
            if(S[i] == S[j]) continue;
            int l = j-i;

            set<char> s;
            s.insert('R');
            s.insert('G');
            s.insert('B');
            s.erase(S[i]);
            s.erase(S[j]);
            char a = *s.begin();
            if(j+l < N && S[j+l] == a){
                ans--;
            }
          switch(a){
                case 'R':
                    ans += R[j];
                    break;
                case 'G':
                    ans += G[j];
                    break;
                case 'B':
                    ans += B[j];
                    break;
            }
        }
    }
    cout << ans << endl;
}



signed main() {
 
    solve();
 
    return 0;
}


Measured memory_bytes: 3692