#include <bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
template<typename T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T, typename U> using ordered_map = tree<T, U, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
typedef long long ll;
typedef long double ld;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<ll> vll;
#define FOR(i,j,k,in) for(int i=(j); i < (k);i+=in)
#define FORD(i,j,k,in) for(int i=(j); i >=(k);i-=in)
#define REP(i,b) FOR(i,0,b,1)
#define REPD(i,b) FORD(i,b,0,1)
#define pb push_back
#define mp make_pair
#define ff first
#define ss second
#define all(x) begin(x), end(x)
#define MANY_TESTS int tcase; cin >> tcase; while(tcase--)
const double EPS = 1e-9;
const int MOD = 1e9+7;
const ll INFF = 1e18;
const int INF = 1e9;
const ld PI = acos((ld)-1);
const vi dy = {1, 0, -1, 0, -1, 1, 1, -1};
const vi dx = {0, 1, 0, -1, -1, 1, -1, 1};
void DBG(){cout << "]" << endl;}
template<typename T, typename ...U> void DBG(const T& head, const U... args){ cout << head << "; "; DBG(args...); }
#define dbg(...) cout << "Line(" << __LINE__ << ") -> [" << #__VA_ARGS__ << "]: [", DBG(__VA_ARGS__);
#define chk() cout << "Check at line(" << __LINE__ << ") hit." << endl;
template<class T, unsigned int U>
ostream& operator<<(ostream& out, const array<T, U> &v){out << "["; REP(i, U) out << v[i] << ", "; out << "]"; return out;}
template <class T, class U>
ostream& operator<<(ostream& out, const pair<T, U> &par) {out << "[" << par.first << ";" << par.second << "]"; return out;}
template <class T>
ostream& operator<<(ostream& out, const set<T> &cont) { out << "{"; for( const auto &x:cont) out << x << ", "; out << "}"; return out; }
template <class T, class U>
ostream& operator<<(ostream& out, const map<T, U> &cont) {out << "{"; for( const auto &x:cont) out << x << ", "; out << "}"; return out; }
template<class T>
ostream& operator<<(ostream& out, const vector<T> &v){ out << "["; REP(i, v.size()) out << v[i] << ", "; out << "]"; return out;}
template<class T>
istream& operator>>(istream& in, vector<T> &v){ for(auto &x : v) in >> x; return in; }
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL); cout.tie(NULL);
int n;
string s;
cin >> n >> s;
int red = 0;
int green = 0;
int yellow = 0;
vector<int> rpos, gpos, ypos;
vector<int> sr(n, 0), sg(n, 0), sy(n, 0);
REP(i, n){
REP(j, i){
if(s[j] == 'R')
sr[i]++;
else if(s[j] == 'G')
sg[i]++;
else
sy[i]++;
}
}
REP(i, n){
if(s[i] == 'R'){
red++;
rpos.pb(i);
}
else if(s[i] == 'G'){
green++;
gpos.pb(i);
}
else{
yellow++;
ypos.pb(i);
}
}
const int mx = 405;
int dp[mx][mx][mx][3];
REP(i, red + 2) REP(j, green + 2) REP(k, yellow + 2) REP(g, 3) dp[i][j][k][g] = INF;
dp[0][0][0][0] = dp[0][0][0][1] = dp[0][0][0][2] = 0;
REP(i, red + 1){
REP(j, green + 1){
REP(k, yellow + 1){
REP(g, 3){
int placed;
// place red
if(i < red && g != 0){
int id = rpos[i];
placed = max(0, sg[id] - j) + max(0, sy[id] - k);
dp[i + 1][j][k][0] = min(dp[i + 1][j][k][0], dp[i][j][k][g] + placed);
}
if(j < green && g != 1){
int id = gpos[j];
placed = max(0, sr[id] - i) + max(0, sy[id] - k);
dp[i][j + 1][k][1] = min(dp[i][j + 1][k][1], dp[i][j][k][g] + placed);
}
if(k < yellow && g != 2){
int id = ypos[k];
placed = max(0, sr[id] - i) + max(0, sg[id] - j);
dp[i][j][k + 1][2] = min(dp[i][j][k + 1][2], dp[i][j][k][g] + placed);
}
}
}
}
}
int ans = INF;
REP(i, 3)
ans = min(ans, dp[red][green][yellow][i]);
if(ans < INF)
cout << ans << "\n";
else
cout << -1 << "\n";
return 0;
}
