#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
#define fi first
#define se second
#define vi vector<int>
#define vll vector<long long>
#define pii pair<int, int>
#define pll pair<long long, long long>
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define all(x) (x).begin(), (x).end()
#define sz(x) (int)(x).size()
mt19937 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
mt19937_64 rng64(chrono::high_resolution_clock::now().time_since_epoch().count());
inline int rand(int l,int r){return uniform_int_distribution<int>(l, r)(rng);}
inline ll rand(ll l,ll r){return uniform_int_distribution<ll>(l, r)(rng64);}
#ifdef DEBUG
auto&operator<<(auto&o,pair<auto,auto>p){return o<<"("<<p.first<<", "<<p.second<<")";}
auto operator<<(auto&o,auto x)->decltype(x.end(),o){o<<"{";int i=0;for(auto e:x)o<<","+!i++<<e;return o<<"}";}
#define debug(X...)cerr<<"["#X"]: ",[](auto...$){((cerr<<$<<"; "),...)<<endl;}(X)
#else
#define debug(...){}
#endif
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);
}
};
struct pair_hash{
size_t operator()(const pair<int,int>&x)const{
return hash<long long>()(((long long)x.first)^(((long long)x.second)<<32));
}
};
const int MAXN = 407;
const int INF = 1e9;
int suf[MAXN][3];
int dp[MAXN][3][MAXN][MAXN];//pozycja, ostatni kolor, ile0, ile1
vi Pos[3];
int A[MAXN];
int n;
string s;
int countInv(int pos, int nkol, int c0, int c1, int c2){
//cerr << nkol << ' ' << sz(Pos[nkol]) << ' ' << c0 << ' ' << c1 << ' ' << c2 << ' ' << pos << '\n';
int pre = Pos[nkol][(nkol == 0 ? c0 : (nkol == 1 ? c1 : c2))];
int ret = 0;
for(int i = 0; i < 3; i++){
if(i == nkol){
continue;
}
ret += max(0, -(suf[1][i] - suf[pre][i] - (i == 0 ? c0 : (i == 1 ? c1 : c2))));
}
return ret;
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(NULL);
cout.tie(NULL);
cin >> n;
cin >> s;
for(int i = 1; i <= n; i++){
if(s[i - 1] == 'R'){
A[i] = 1;
}else if(s[i - 1] == 'G'){
A[i] = 2;
}
Pos[A[i]].pb(i);
}
for(int i = n; i >= 1; i--){
for(int j = 0; j < 3; j++){
for(int i1 = 0; i1 <= n; i1++){
for(int i2 = 0; i2 <= n; i2++){
dp[i][j][i1][i2] = INF;
}
}
suf[i][j] = suf[i + 1][j];
if(A[i] == j){
suf[i][j]++;
}
}
}
for(int i = 0; i < n; i++){
for(int c = 0; c < 3; c++){
for(int c0 = 0; c0 <= suf[1][0]; c0++){
for(int c1 = 0; c1 <= suf[1][1]; c1++){
if(dp[i][c][c0][c1] == INF || i < c0 + c1){
continue;
}
for(int cn = 0; cn < 3; cn++){
if(i > 0 && cn == c){
continue;
}
if(cn == 0 && c0 + 1 > suf[1][0]){
continue;
}
if(cn == 1 && c1 + 1 > suf[1][1]){
continue;
}
if(cn == 2 && i - c0 - c1 + 1 > suf[1][2]){
continue;
}
dp[i + 1][cn][c0 + (cn == 0)][c1 + (cn == 1)] = min(dp[i + 1][cn][c0 + (cn == 0)][c1 + (cn == 1)], dp[i][c][c0][c1] + countInv(i + 1, cn, c0, c1, i - c0 - c1));
}
}
}
}
}
int ans = min({dp[n][0][suf[1][0]][suf[1][1]], dp[n][1][suf[1][0]][suf[1][1]], dp[n][2][suf[1][0]][suf[1][1]]});
if(ans >= INF){
cout << "-1\n";
}else{
cout << ans << '\n';
}
return 0;
}
