#pragma GCC optimize ("O3")
#include <iostream>
#include <iomanip>
#include <istream>
#include <ostream>
#include <sstream>
#include <iterator>
#include <vector>
#include <algorithm>
#include <queue>
#include <deque>
#include <list>
#include <stack>
#include <map>
#include <unordered_map>
#include <set>
#include <bitset>
#include <utility>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <string>
#include <ctime>
#include <cctype>
#include <cstdlib>
#define IINF 100000000
#define INF 1000000000000000000
#define MOD 1000000007
#define mod 1000000007
#define REP(i, a, n) for (ll i = a; i < (ll)(n); i++)
#define REPE(i, a, n) for (ll i = a; i <= (ll)(n); i++)
#define Endl endl
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define mt make_tuple
#define eb emplace_back
#define mmax(x,y)(x>y?x:y)
#define mmin(x,y)(x<y?x:y)
#define chmax(x,y) x=mmax(x,y)
#define chmin(x,y) x=mmin(x,y)
#define all(x) (x).begin(),(x).end()
#define siz(x) (ll)(x).size()
#define PI acos(-1.0)
using namespace std;
typedef long long int ll;
typedef unsigned long long int ull;
typedef long double ld;
typedef pair<int,int>Pin;
typedef pair<ll,ll>Pll;
template<class T> using V=vector<T>;
long long GCD(long long a, long long b) {return b?GCD(b,a%b):a;}
long long LCM(long long a, long long b) {return a/GCD(a,b)*b;}
int dx[4]={-1,0,1,0};
int dy[4]={0,-1,0,1};
int ddx[8]={-1,0,1,0,1,1,-1,-1};
int ddy[8]={0,-1,0,1,1,-1,1,-1};
ll cmp(pair<ll,ll>a,pair<ll,ll> b){
        if(a.se!=b.se)
        return a.se<b.se;
        else
        return a.fi<b.fi;
}
//----------------------------------------------------------------------
int R[4040],B[4040],G[4040];
//----------------------------------------------------------------------
int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    //-------------------------------  
    //ll begin_time=clock();
    //-------------------------------
    /*ll n;cin>>n;
    V<ll>use;
    map<ll,ll>m;ll ans=0;
    for(ll i=1;i<=n;i++){
        if(i%3!=0&&i%5!=0)ans+=i;
    }
    cout<<ans<<endl;

    int k;cin>>k;
    int ans=0;
    for(int a=1;a<=k;a++){
        for(int b=1;b<=k;b++){
            for(int c=1;c<=k;c++){
                ans+=GCD(a,GCD(b,c));
            }
        }
    }
    cout<<ans<<endl;*/


    int n;cin>>n;
    string s;cin>>s;
    ll ans=0;
    /*for(int i=0;i<n;i++){
        for(int j=i+1;j<n;j++){
            for(int k=j+1;k<n;k++){
                if(j-i==k-j)continue;
                if(s[j]==s[i]||s[j]==s[k]||s[i]==s[k])continue;
                ans++;
            }
        }
    }*/
    if(s[0]=='R')R[0]++;
    else if(s[0]=='B')B[0]++;
    else G[0]++;
    for(int i=1;i<n;i++){
        if(s[i]=='R')R[i]++;
        else if(s[i]=='B')B[i]++;
        else G[i]++;
        R[i]+=R[i-1];
        B[i]+=B[i-1];
        G[i]+=G[i-1];
    }
    for(int i=0;i<n;i++){
        for(int j=i+1;j<n;j++){
            if(s[i]==s[j])continue;
            int c=j-i;
            int k=j+c;
            if(s[i]=='R'){
                if(s[j]=='G'){
                    ans+=B[n-1]-B[j];
                    if(k<n){
                        if(s[k]=='B')ans--;
                    }
                }
                else if(s[j]=='B'){
                    ans+=G[n-1]-G[j];
                    if(k<n){
                        if(s[k]=='G')ans--;
                    }
                }
            }
            else if(s[i]=='B'){
                if(s[j]=='G'){
                    ans+=R[n-1]-R[j];
                    if(k<n){
                        if(s[k]=='R')ans--;
                    }
                }
                else if(s[j]=='R'){
                    ans+=G[n-1]-G[j];
                    if(k<n){
                        if(s[k]=='G')ans--;
                    }
                }
            }
            else if(s[i]=='G'){
                if(s[j]=='B'){
                    ans+=R[n-1]-R[j];
                    if(k<n){
                        if(s[k]=='R')ans--;
                    }
                }
                else if(s[j]=='R'){
                    ans+=B[n-1]-B[j];
                    if(k<n){
                        if(s[k]=='B')ans--;
                    }
                }
            }
        }
        //cout<<ans<<endl;
    }
    cout<<ans<<endl;
    //-------------------------------  
    //ll end_time=clock();cout<<"time="<<end_time-begin_time<<"ms"<<endl;
    //-------------------------------
    return 0;
}
//----------------------------------------------------------------------




































Measured memory_bytes: 3668