/*
* Author: xynex
* Created: 24.12.2021 17:20
* Why am I so stupid? :c
* Slishkom slab
*/
#include <bits/stdc++.h>
// #pragma GCC optimize("inline")
// #pragma GCC optimize("-fgcse,-fgcse-lm")
// #pragma GCC optimize("-ftree-pre,-ftree-vrp")
// #pragma GCC optimize("-ffast-math")
// #pragma GCC optimize("-fipa-sra")
// #pragma GCC optimize("-fpeephole2")
// #pragma GCC optimize("-fsched-spec")
// #pragma GCC optimize("Ofast,no-stack-protector")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
// #pragma GCC optimize("unroll-loops")
using namespace std;
#define ll long long
#define dl double long
#define ull unsigned long long
#define pr pair
#define vt vector
#define ff first
#define ss second
#define mp make_pair
#define sz(a) (int)a.size()
#define pb push_back
#define pf push_front
#define popB pop_back
#define popF pop_front
#define bit_count __builtin_popcount
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define sp(x) fixed << setprecision(x)
template<typename T> T get_rand(T l, T r) {
random_device rd;
mt19937 gen(rd());
return uniform_int_distribution<T>(l, r)(gen);
}
template<typename T> T lcm(T a, T b) { return a * (b / __gcd(a, b)); }
template<class A> void read(vt<A>& v);
template<class A, size_t S> void read(array<A, S>& a);
template<class T> void read(T& x) { cin >> x; }
void read(double& d) { string t; read(t); d = stod(t); }
void read(long double& d) { string t; read(t); d = stold(t); }
template<class H, class... T> void read(H& h, T&... t) { read(h); read(t...); }
template<class A> void read(vt<A>& x) { for (auto& a : x) read(a); }
template<class A, size_t S> void read(array<A, S>& x) { for (auto& a : x) read(a); }
string to_string(char c) { return string(1, c); }
string to_string(bool b) { return b ? "true" : "false"; }
string to_string(const char* s) { return string(s); }
string to_string(string s) { return s; }
string to_string(vt<bool> v) { string res; for (int i = 0; i < sz(v); ++i) res += char('0' + v[i]); return res; }
template<size_t S> string to_string(bitset<S> b) { string res; for (int i = 0; i < S; ++i) res += char('0' + b[i]); return res; }
template<class T> string to_string(T v) { bool f = 1; string res; for (auto x : v) { if (!f) res += ' '; f = 0; res += to_string(x); } return res; }
template<class A> void write(A x) { cout << to_string(x); }
template<class H, class... T> void write(const H& h, const T&... t) { write(h); write(t...); }
void print() { write("\n"); }
template<class H, class... T> void print(const H& h, const T&... t) { write(h); if (sizeof...(t)) write(' '); print(t...); }
void freop(string s) {
freopen((s + ".in").c_str(), "r", stdin);
freopen((s + ".out").c_str(), "w", stdout);
}
const int MOD = 1e9 + 7;
const int N = 3e6 + 5;
const ll INF = 9e18;
const int M = 4e2 + 1;
const dl pi = acos(-1);
const dl eps = 1e-12;
const int sq = 700;
int dx[] = {-1, 0, 1, 0};
int dy[] = {0, -1, 0, 1};
/* ll vs int*/
void precalc() {
}
int n;
string s;
int get(char a) {
if(a == 'R') return 0;
if(a == 'G') return 1;
if(a == 'Y') return 2;
}
struct ost {
int x, y, z;
};
vt<ost > el[3];
int cnt[3];
int dp[M][M][M][3];
void solve() {
read(n);
read(s);
int x = 0, y = 0, z = 0;
for(int i = 0; i < n; ++i) {
cnt[get(s[i])]++;
ost cur;
cur.x = cnt[0];
cur.y = cnt[1];
cur.z = cnt[2];
el[get(s[i])].pb(cur);
}
x = sz(el[0]);
y = sz(el[1]);
z = sz(el[2]);
for(int i = 0; i <= x; ++i) {
for(int j = 0; j <= y; ++j) {
for(int k = 0; k <= z; ++k) dp[i][j][k][0] = dp[i][j][k][1] = dp[i][j][k][2] = MOD;
}
}
// print(x, y, z);
dp[0][0][0][0] = 0, dp[0][0][0][1] = 0, dp[0][0][0][2] = 0;
for(int i = 0; i <= x; ++i) {
for(int j = 0; j <= y; ++j) {
for(int k = 0; k <= z; ++k) {
if(i) {
dp[i][j][k][0] = min(dp[i - 1][j][k][1], dp[i - 1][j][k][2]) + max(0, j - el[0][i - 1].y) + max(0, k - el[0][i - 1].z);
}
if(j) {
dp[i][j][k][1] = min(dp[i][j - 1][k][0], dp[i][j - 1][k][2]) + max(0, i - el[1][j - 1].x) + max(0, k - el[1][j - 1].z);
}
if(k) {
dp[i][j][k][2] = min(dp[i][j][k - 1][0], dp[i][j][k - 1][1]) + max(0, i - el[2][k - 1].x) + max(0, j - el[2][k - 1].y);
}
}
}
}
int ans = MOD;
for(int i = 0; i < 3; ++i) ans = min(ans, dp[x][y][z][i]);
if(ans == MOD) ans = -1;
print(ans);
}
int main() {
//freop("");
ios::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int t = 1;
//read(t);
precalc();
for (int i = 1; i <= t; ++i) {
//write("Case #" + to_string(i) + ": ");
solve();
}
return 0;
}
