#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/trie_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> pbds;
typedef trie<string, null_type, trie_string_access_traits<>, pat_trie_tag, trie_prefix_search_node_update> pref_trie;
typedef unsigned long long BITMASK; typedef long long ll; typedef long double ld; typedef pair<int, int> pii; typedef pair<double, double> pdd; typedef pair<long long, long long> pll; typedef cc_hash_table<int, int, hash<int>> intHashTable;
#define has_bit(bit_mask, x) ((bit_mask) & (1ULL << (x)))
#define turn_on_bit(bit_mask, x) (bit_mask |= (1ULL << (x)))
#define turn_off_bit(bit_mask, x) (bit_mask &= (~(1ULL << (x))))
#define smallest_on_bit(bit_mask) (__builtin_ctzint((bit_mask) & (-(bit_mask))))
#define CLOCK_START clock_t chrono_clk_beg = clock()
#define CLOCK_END clock_t chrono_clk_end = clock(); cerr << (double(chrono_clk_end - chrono_clk_beg) / CLOCKS_PER_SEC) << " sec"
#define bug(args ...) cerr << __LINE__ << ">> ", err(new istringstream(string(#args)), args), cerr << '\n'
#define decToBin(name, num) string name = bitset<8>(num).to_string();
#define binToDec(name, binary) unsigned long name = bitset<8>(binary).to_ulong();
#define radToDeg(name, radians) double name = radians * 180 / M_PI;
#define all(x) x.begin(), x.end()
#define removeDuplicates(vec) sort(vec.begin(), vec.end()); vec.erase(unique(vec.begin(), vec.end()), vec.end())
#define printMat(mat) for (auto &x : mat) { for (auto &y : x) { if (y < INF) cout << setw(7) << y; else cout << setw(7) << "-"; } cout << '\n'; }
#define fillMat(mat, n, m, x) for (int i = 0; i < n; ++i) for (int j = 0; j < m; ++j) mat[i][j] = x;
#define printArr(arr) for (auto &x : arr) cout << x << " "; cout << '\n';
#define arrReplace(arr, from, to) for (auto &x : arr) if (x == from) x = to;
#define INF (1LL<<30)
#define INF32 (1LL<<62)
#define F first
#define S second
#define M_PI 3.14159265358979323846
#define MOD 1000000007
#define int ll
struct custom_hash { static uint64_t splitmix64(uint64_t x) { x += 0x9e3779b97f4a7c15; x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9; x = (x ^ (x >> 27)) * 0x94d049bb133111eb; return x ^ (x >> 31); } size_t operator()(uint64_t x) const { static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count(); return splitmix64(x + FIXED_RANDOM); } };
inline int powMod(int a, int b) { int x = 1; while (b > 0) { if (b&1) x = (x*a) % MOD; a = (a*a) % MOD; b >>= 1; } return x; }
inline int multiply(int x, int y) { return ((x % MOD) * (y % MOD)) % MOD; }
inline int divide(int x, int y) { return ((x % MOD) * powMod(y % MOD, MOD-2)) % MOD; }
inline int ceil(int a, int b) { return (a+b-1)/b; }
int gcd (int a, int b) { while (b) { a %= b; swap(a, b); } return a; }
int lcm (int a, int b) { return a / gcd(a, b) * b; }
int inverseMod(int a, int m) { a = a % m; for (ll x = 1; x < m; x++) if ((a * x) % m == 1) return x; return -1; }
void err(istringstream *iss) {} template<typename T, typename ... Args> void err(istringstream *iss, const T &_val, const Args & ... args) { string _name; *iss >> _name; if (_name.back()==',') _name.pop_back(); cerr << _name << " = " << _val << "; ", err(iss, args ...); }
int str_replace(string& str, const string& from, const string& to, int limit = -1) { if(from.empty()) return 0; size_t start_pos = 0; int cnt = 0; while((start_pos = str.find(from, start_pos)) != std::string::npos) { str.replace(start_pos, from.length(), to); start_pos += to.length(); cnt++; if (cnt == limit) break; } return cnt; }

signed main()
{
    ios_base::sync_with_stdio(false); cin.tie(NULL);
    int n; cin >> n;
    string str; cin >> str;
    vector<int> pos[3];
    set<int> rec;
    for (int i = 0; i < n; ++i)
    {
        if (str[i] == 'R') pos[0].push_back(i);
        else if (str[i] == 'G') pos[1].push_back(i);
        else if (str[i] == 'B') { pos[2].push_back(i); rec.insert(i); }
    }
    int ans = 0;
    for (auto x : pos[0])
    {
        for (auto y : pos[1])
        {
            int p = (2*y - x);
            int q = (2*x - y);
            int r = ((x+y) % 2 == 0) ? (x+y)/2 : -1;
            ans += (pos[2].size() - rec.count(p) - rec.count(q) - rec.count(r));
        }
    }
    cout << ans << '\n';
    return 0;
}

Measured memory_bytes: 3692