// #pragma GCC target("avx2")
// #pragma GCC optimize("O3")
// #include <x86intrin.h>
#include <bits/stdc++.h>
#include <chrono>
#include <random>
// @author: Vlapos
namespace operators
{
template <typename T1, typename T2>
std::istream &operator>>(std::istream &in, std::pair<T1, T2> &x)
{
in >> x.first >> x.second;
return in;
}
template <typename T1, typename T2>
std::ostream &operator<<(std::ostream &out, std::pair<T1, T2> x)
{
out << x.first << " " << x.second;
return out;
}
template <typename T1>
std::istream &operator>>(std::istream &in, std::vector<T1> &x)
{
for (auto &i : x)
in >> i;
return in;
}
template <typename T1>
std::ostream &operator<<(std::ostream &out, std::vector<T1> &x)
{
for (auto &i : x)
out << i << " ";
return out;
}
template <typename T1>
std::ostream &operator<<(std::ostream &out, std::set<T1> &x)
{
for (auto &i : x)
out << i << " ";
return out;
}
}
// name spaces
using namespace std;
using namespace operators;
// end of name spaces
// defines
#define ll long long
#define ull unsigned long long
#define pb push_back
#define mp make_pair
#define pii pair<int, int>
#define pll pair<ll, ll>
#define f first
#define s second
#define uint unsigned int
#define all(vc) vc.begin(), vc.end()
// end of defines
// usefull stuff
void boost()
{
ios_base ::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
}
inline int getbit(int &x, int &bt) { return (x >> bt) & 1; }
const int dx4[4] = {-1, 0, 0, 1};
const int dy4[4] = {0, -1, 1, 0};
const int dx8[8] = {-1, -1, -1, 0, 0, 1, 1, 1};
const int dy8[8] = {-1, -0, 1, -1, 1, -1, 0, 1};
const ll INF = (1e18) + 500;
const int BIG = (1e9) * 2 + 100;
const int MAXN = (1e5) + 5;
const int MOD7 = (1e9) + 7;
const int MOD9 = (1e9) + 9;
const uint MODFFT = 998244353;
// #define int ll
struct test
{
void solve(int testcase)
{
boost();
int n;
cin >> n;
string s;
cin >> s;
vector<int> a(n);
map<char, int> ID;
ID['R'] = 0;
ID['G'] = 1;
ID['Y'] = 2;
for (int i = 0; i < n; ++i)
{
a[i] = ID[s[i]];
}
vector<vector<int>> pref(3, vector<int>(n)), pos(3, vector<int>(n));
vector<int> cnt(3);
for (int i = 0; i < n; ++i)
{
pref[a[i]][i]++;
pos[a[i]][cnt[a[i]]] = i;
cnt[a[i]]++;
if (i)
for (int j = 0; j < 3; ++j)
pref[j][i] += pref[j][i - 1];
}
auto cost = [&](int cur, int id, vector<int> &C) -> int
{
int res = 0;
for (int j = 0; j < 3; ++j)
if (j != id)
res += max(0, pref[j][pos[id][cur]] - C[j]);
return res;
};
int dp[cnt[0] + 1][cnt[1] + 1][cnt[2] + 1][3];
for (int r = 0; r <= cnt[0]; ++r)
for (int g = 0; g <= cnt[1]; ++g)
for (int b = 0; b <= cnt[2]; ++b)
dp[r][g][b][0] = dp[r][g][b][1] = dp[r][g][b][2] = BIG;
dp[0][0][0][0] = dp[0][0][0][1] = dp[0][0][0][2] = 0;
for (int r = 0; r <= cnt[0]; ++r)
for (int g = 0; g <= cnt[1]; ++g)
for (int b = 0; b <= cnt[2]; ++b)
for (int last = 0; last < 3; ++last)
{
vector<int> BF = {r, g, b};
if (r != cnt[0] and last != 0)
dp[r + 1][g][b][0] = min(dp[r + 1][g][b][0], dp[r][g][b][last] + cost(r, 0, BF));
if (g != cnt[1] and last != 1)
dp[r][g + 1][b][1] = min(dp[r][g + 1][b][1], dp[r][g][b][last] + cost(g, 1, BF));
if (b != cnt[2] and last != 2)
dp[r][g][b + 1][2] = min(dp[r][g][b + 1][2], dp[r][g][b][last] + cost(b, 2, BF));
}
int res = min({dp[cnt[0]][cnt[1]][cnt[2]][0], dp[cnt[0]][cnt[1]][cnt[2]][1], dp[cnt[0]][cnt[1]][cnt[2]][2]});
if (res != BIG)
cout << res;
else
cout << "-1\n";
}
};
main()
{
boost();
int q = 1;
// cin >> q;
for (int i = 0; i < q; i++)
{
test t;
t.solve(i);
}
return 0;
}
//[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]//
// //
// Coded by Der_Vlἀpos //
// //
//[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]//
