#include <bits/stdc++.h>
#define ll long long
#define fi first
#define se second
#define sz(x) (int)x.size()
#define ALL(v) v.begin(),v.end()
#define MASK(k) (1LL << (k))
#define BIT(x, i) (((x) >> (i)) & 1)
#define oo (ll)1e18
#define INF (ll)1e9
#define MOD (ll)(1e9 + 7)
using namespace std;
template<class T1, class T2>
bool maximize(T1 &a, T2 b){if(a < b){a = b; return true;} return false;}
template<class T1, class T2>
bool minimize(T1 &a, T2 b){if(a > b){a = b; return true;} return false;}
template<class T1, class T2>
void add(T1 &a, T2 b){a += b; if(a >= MOD) a -= MOD;}
template<class T1, class T2>
void sub(T1 &a, T2 b){a -= b; if(a < 0) a += MOD;}
template<class T>
void cps(T &v){sort(ALL(v)); v.resize(unique(ALL(v)) - v.begin());}
const int MAX = 403;
int N;
string s;
int pre[3][MAX];
vector<int> pos[MAX];
vector<vector<vector<vector<int>>>> dp;
int convert(char kt){
if(kt == 'R') return 0;
if(kt == 'G') return 1;
if(kt == 'Y') return 2;
return 2;
}
void solve(){
cin >> N >> s;
s = " " + s;
for(int i = 1; i <= N; i++){
int t = convert(s[i]);
pos[t].push_back(i);
for(int j = 0; j < 3; j++) pre[j][i] = pre[j][i - 1];
pre[t][i]++;
}
dp.resize(sz(pos[0]) + 1, vector<vector<vector<int>>>(sz(pos[1] ) + 1, vector<vector<int>>(sz(pos[2]) + 1, vector<int>(3, INF))));
for(int i = 0; i < 3; i++) dp[0][0][0][i] = 0;
for(int i = 0; i <= sz(pos[0]); i++){
for(int j = 0; j <= sz(pos[1]); j++){
for(int k = 0; k <= sz(pos[2]); k++){
for(int p = 0; p < 3; p++) if(dp[i][j][k][p] < INF){
if(p != 0 && i < sz(pos[0])){
int cost1 = max(pre[1][pos[0][i]] - j, 0);
int cost2 = max(pre[2][pos[0][i]] - k, 0);
minimize(dp[i + 1][j][k][0], dp[i][j][k][p] + cost1 + cost2);
}
if(p != 1 && j < sz(pos[1])){
int cost1 = max(pre[0][pos[1][j]] - i, 0);
int cost2 = max(pre[2][pos[1][j]] - k, 0);
minimize(dp[i][j + 1][k][1], dp[i][j][k][p] + cost1 + cost2);
}
if(p != 2 && k < sz(pos[2])){
int cost1 = max(pre[0][pos[2][k]] - i, 0);
int cost2 = max(pre[1][pos[2][k]] - j, 0);
minimize(dp[i][j][k + 1][2], dp[i][j][k][p] + cost1 + cost2);
}
}
}
}
}
int ans = INF;
for(int i = 0; i < 3; i++) minimize(ans, dp[sz(pos[0])][sz(pos[1])][sz(pos[2])][i]);
if(ans >= INF) ans = -1;
cout << ans << '\n';
}
//nhan xet: vi tri cung mau nhau sau khi sap xep van nam theo 1 thu tu
//ta dua lan luot cac mau va duoi day da tao
int main(){
ios_base::sync_with_stdio(0); cin.tie(0);
// freopen("deggraph.inp", "r", stdin);
// freopen("deggraph.out", "w", stdout);
int t = 1;
// cin >> t;
while(t--){
solve();
}
return 0;
}
