#include <iostream>
#include <iomanip>
#include <string>
#include <stack>
#include <vector>
#include <complex>
#include <math.h>
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <iterator>
#include <map>
#include <set>
#include <queue>
#include <list>
#include <regex>
#include <limits>
#include <time.h>
#include <cstdint>
using namespace std;
using pii  = pair<int,int>;
using ll=long long;
using ld=long double;
#define pb push_back
#define mp make_pair
#define sc second
#define fr first
#define stpr setprecision
#define cYES cout<<"YES"<<endl
#define cNO cout<<"NO"<<endl
#define cYes cout<<"Yes"<<endl
#define cNo cout<<"No"<<endl
#define rep(i,n) for(ll i=0;i<(n);++i)
#define Rep(i,a,b) for(ll i=(a);i<(b);++i)
#define rrep(i,n) for(ll i=n-1;i>=0;i--)
#define rRep(i,a,b) for(ll i=a;i>=b;i--)
#define crep(i) for(char i='a';i<='z';++i)
#define psortsecond(A,N) sort(A,A+N,[](const pii &a, const pii &b){return a.second<b.second;});
#define ALL(x) (x).begin(),(x).end()
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define endl '\n'
int ctoi(const char c){
  if('0' <= c && c <= '9') return (c-'0');
  return -1;
}
ll gcd(ll a,ll b){return (b == 0 ? a : gcd(b, a%b));}
ll lcm(ll a,ll b){return a*b/gcd(a,b);}
constexpr ll MOD=1000000007;
constexpr ll INF=1000000011;
constexpr ll MOD2=998244353;
constexpr ll LINF = 1001002003004005006ll;
constexpr ld EPS=10e-8;
template <class T, class U> inline bool chmax(T& lhs, const U& rhs) { if (lhs < rhs) { lhs = rhs; return 1; } return 0; }
template <class T, class U> inline bool chmin(T& lhs, const U& rhs) { if (lhs > rhs) { lhs = rhs; return 1; } return 0; }
template<typename T> istream& operator>>(istream& is,vector<T>& v){for(auto&& x:v)is >> x;return is;}
template<typename T,typename U> istream& operator>>(istream& is, pair<T,U>& p){ is >> p.first; is >> p.second; return is;}
template<typename T,typename U> ostream& operator>>(ostream& os, const pair<T,U>& p){ os << p.first << ' ' << p.second; return os;}
template<class T> ostream& operator<<(ostream& os, vector<T>& v){
  for(auto i=begin(v); i != end(v); ++i){
    if(i !=begin(v)) os << ' ';
    os << *i;
  }
  return os;
}

template <std::uint_fast64_t Modulus> class modint {
  using u64 = std::uint_fast64_t;

public:
  u64 a;

  constexpr modint(const u64 x = 0) noexcept : a(x % Modulus) {}
  constexpr u64 &value() noexcept { return a; }
  constexpr const u64 &value() const noexcept { return a; }
  constexpr modint operator+(const modint rhs) const noexcept {
    return modint(*this) += rhs;
  }
  constexpr modint operator-(const modint rhs) const noexcept {
    return modint(*this) -= rhs;
  }
  constexpr modint operator*(const modint rhs) const noexcept {
    return modint(*this) *= rhs;
  }
  constexpr modint operator/(const modint rhs) const noexcept {
    return modint(*this) /= rhs;
  }
  constexpr modint &operator+=(const modint rhs) noexcept {
    a += rhs.a;
    if (a >= Modulus) {
      a -= Modulus;
    }
    return *this;
  }
  constexpr modint &operator-=(const modint rhs) noexcept {
    if (a < rhs.a) {
      a += Modulus;
    }
    a -= rhs.a;
    return *this;
  }
  constexpr modint &operator*=(const modint rhs) noexcept {
    a = a * rhs.a % Modulus;
    return *this;
  }
  constexpr modint &operator/=(modint rhs) noexcept {
    u64 exp = Modulus - 2;
    while (exp) {
      if (exp % 2) {
        *this *= rhs;
      }
      rhs *= rhs;
      exp /= 2;
    }
    return *this;
  }
};

string S;
ll N,sr[4007],sg[4007],sb[4007];

int main(){
  cin >> N >> S;
  ll R=0,G=0,B=0;
  rep(i,N){
    if(S[i]=='R'){
      R++;
    }
    else if(S[i]=='G'){
      G++;
    }
    else{
      B++;
    }
    sr[i]=R;sg[i]=G;sb[i]=B;
  }
  ll ANS=0;
  rep(i,N-2){
    Rep(j,i+1,N-1){
      if(S[i]=='R'){
        if(S[j]=='B'){
          ANS+=sg[N-1]-sg[j];
        }
        if(S[j]=='G'){
          ANS+=sb[N-1]-sb[j];
        }
      }
      if(S[i]=='G'){
        if(S[j]=='B'){
          ANS+=sr[N-1]-sr[j];
        }
        if(S[j]=='R'){
          ANS+=sb[N-1]-sb[j];
        }
      }
      if(S[i]=='B'){
        if(S[j]=='R'){
          ANS+=sg[N-1]-sg[j];
        }
        if(S[j]=='G'){
          ANS+=sr[N-1]-sr[j];
        }
      }
    }
  }
  for(ll i=1;i*2<N;i++){
    rep(j,N){
      if(j+2*i>=N){
        break;
      }
      if(S[j]!=S[j+i] && (S[j]!=S[j+i+i] && S[j+i]!=S[j+i+i] )){
        ANS--;
      }
    }
  }
  cout << ANS << endl;
}

Measured memory_bytes: 3740