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

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rept(i,k,n) for(int i = (k); i < (int)(n); i++)

#define mod 1000000007
typedef long long ll;

int main() {
    int n;
    string s;
    cin>>n>>s;
    vector<vector<int>> rgb(3,vector<int>(n+1,0));
    for(int i=n-1;i>=0;i--){
        rgb[0][i]=rgb[0][i+1];rgb[1][i]=rgb[1][i+1];rgb[2][i]=rgb[2][i+1];
        if(s[i]=='R') rgb[0][i]++;
        if(s[i]=='G') rgb[1][i]++;
        if(s[i]=='B') rgb[2][i]++;
    }
    ll ans =0;
    map<char,int> mp;
    mp['R']=0;mp['G']=1;mp['B']=2;
    rep(i,n){
        rept(j,i+1,n-1){
            if(s[i]==s[j]) continue;
            int cl = 3-mp[s[i]]-mp[s[j]];
            ans+=rgb[cl][j+1];
            if(j*2-i<=n-1){
                if(mp[s[j*2-i]]==cl) ans--;
            }
        }
    }

    cout<<ans<<endl;
    return 0;
}
