#include <bits/stdc++.h>
#define pb push_back
#define ins isnert
#define pii pair<int,int>
#define ff first
#define ss second
#define loop(i,a,b) for(int i = (a); i < (b); i ++)
#define op(x) cerr << #x << " = " << x << endl;
#define opa(x) cerr << #x << " = " << x << ", ";
#define ops(x) cerr << x;
#define spac cerr << ' ';
#define entr cerr << endl;
#define STL(x) cerr << #x << " : "; for(auto &qwe:x) cerr << qwe << ' '; cerr << endl;
#define ARR(x, nnn) cerr << #x << " : "; loop(qwe,0,nnn) cerr << x[qwe] << ' '; cerr << endl;
#define BAE(x) (x).begin(), (x).end()
using namespace std;
mt19937 RNG(chrono::steady_clock::now().time_since_epoch().count());
int a[405], cnt[3];
vector<int> pos[3];
int dp[405][405][405][3];
bool vis[405][405][405][3];
const int INF = 1e8;
inline void upd(int &x, int y){
x = min(x, y);
}
inline int pos_diff(int ca, int cb, int cc, int last){
int now = ca + cb + cc - 1; // 0-based
if(last == 0){
return abs(now - pos[0][ca]);
}
if(last == 1){
return abs(now - pos[1][cb]);
}
if(last == 2){
return abs(now - pos[2][cc]);
}
}
int cac(int ca, int cb, int cc, int last){
if(ca < 0 || cb < 0 || cc < 0) return INF;
if(ca == 0 && cb == 0 && cc == 0) return 0;
if(last == 0 && ca == 0) return INF;
if(last == 1 && cb == 0) return INF;
if(last == 2 && cc == 0) return INF;
if(vis[ca][cb][cc][last]) return dp[ca][cb][cc][last];
loop(i,0,3){
if(i == last) continue;
int ret = cac(ca-(last == 0), cb-(last == 1), cc-(last == 2), i) + pos_diff(ca, cb, cc, last);
upd(dp[ca][cb][cc][last], ret);
}
return dp[ca][cb][cc][last];
}
int main(){
ios::sync_with_stdio(false); cin.tie(0);
// freopen("in.txt", "r", stdin);
int n; string s;
cin >> n >> s;
loop(i,0,3) pos[i].pb(-999);
loop(i,0,n){
if(s[i] == 'R') a[i] = 0;
else if(s[i] == 'G') a[i] = 1;
else a[i] = 2;
cnt[a[i]]++;
pos[a[i]].pb(i);
}
loop(i,0,cnt[0]+1)
loop(j,0,cnt[1]+1)
loop(k,0,cnt[2]+1)
loop(l,0,3)
dp[i][j][k][l] = INF;
loop(i,0,3){
vis[0][0][0][i] = true;
dp[0][0][0][i] = 0;
}
loop(i,0,3){
if(cnt[i] != 0)
cac(cnt[0], cnt[1], cnt[2], i);
}
int ans = INF;
loop(i,0,3) ans = min(ans, dp[cnt[0]][cnt[1]][cnt[2]][i]);
if(ans == INF) cout << "-1\n";
else cout << (ans >> 1) << '\n';
}
