RGB Triplets

// In The Name of Almighty Allah
#include<bits/stdc++.h>
using namespace std;
#define             fastio              ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define             ff                  first
#define             ss                  second
#define             int                 long long
#define             pb                  push_back
#define             pf                  push_front
#define             pii                 pair<int,int>
#define             print(x)            for(auto it : x) cout<<it<<' '; cout<<'\n';
#define             debug(x)            cout << #x << " --->> " << x << endl;
#define             mod                 1000000007
#define             inf                 1e18
#define             PI                  (acos(-1))
#define             eps                 (1e-8)
#define             clear(x)            memset(x,0,sizeof(x));


main()
{
    fastio;
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif

    int n;

    string s;

    cin >> n;
    cin >> s;

    pair<int, pii>cum[n];

    int ans = 0, r = 0, g = 0, b = 0;

    for (int i = 0; i < n; i++)
    {
        if (s[i] == 'R') r++;
        else if (s[i] == 'B') b++;
        else g++;

        cum[i].ff = r, cum[i].ss.ff = g, cum[i].ss.ss = b;
    }

    for (int i = 0; i < n; i++)
    {
        for (int j = i + 1; j < n - 1; j++)
        {
            if (s[j] == s[i]) continue;

            int k = 2 * j - i;
            // debug(k);
            if ((s[i] == 'R' && s[j] == 'G') || (s[i] == 'G' && s[j] == 'R'))
            {
                ans += (cum[n - 1].ss.ss - cum[j].ss.ss);

                if (k < n && s[k] == 'B')
                    ans--;
            }
            else if ((s[i] == 'R' && s[j] == 'B') || (s[i] == 'B' && s[j] == 'R'))
            {
                ans += (cum[n - 1].ss.ff - cum[j].ss.ff);

                if (k < n && s[k] == 'G')
                    ans--;
            }
            else if ((s[i] == 'G' && s[j] == 'B') || (s[i] == 'B' && s[j] == 'G'))
            {
                ans += (cum[n - 1].ff - cum[j].ff);

                if (k < n && s[k] == 'R')
                    ans--;
            }
        }
    }
    cout << ans << endl;
}