#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define pf push_front
#define mp make_pair
#define fi first
#define se second
#define int long long
#define all(x) (x).begin(), (x).end()
typedef long double ld;
typedef long long ll;
typedef pair<ll,ll> pll;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<bool> vb;
typedef vector<vector<int>> vvi;
typedef vector<vector<bool>> vvb;
typedef vector<vector<ll>> vvll;
typedef vector<string> vs;
typedef vector<vector<string>> vvs;
typedef vector<char> vc;
typedef vector<vector<char>> vvc;
typedef map<int, int> mii;
typedef unordered_map<int, int> umii;
const int mod = 1e9 + 7;
const int inf = INTMAX_MAX;
const bool tc = false;
int n; string s;
const int mxn = 401;
int dp[mxn][mxn][mxn][3];
int rp[mxn], gp[mxn], yp[mxn];
// R, G, Y
// 0, 1, 2
int cnt(int id, int y) {
// cnt < y
// if (!x.size()) return 0;
// if (x[x.size() - 1] < y) return x.size();
// auto it = lower_bound(all(x), y);
// if (*it < y) return x.size();
// if (it - x.begin() != 0) {
// return (it - x.begin());
// } else {
// return 0;
// }
if (y == 0) return 0;
if (id == 0) return rp[y - 1];
else if (id == 1) return gp[y - 1];
else return yp[y - 1];
}
void cmn(int &x, int y) {x = min(x, y);}
void pre() {
for (int i = 0; i <= n; i++) for (int j = 0; j <= n; j++) for (int k = 0; k <= n; k++) for (int l = 0; l < 3; l++) dp[i][j][k][l] = 1e18;
if (s[0] == 'R') rp[0]++;
else if (s[0] == 'G') gp[0]++;
else yp[0]++;
for (int i = 1; i < n; i++) {
rp[i] = rp[i - 1] += (s[i] == 'R');
gp[i] = gp[i - 1] += (s[i] == 'G');
yp[i] = yp[i - 1] += (s[i] == 'Y');
}
}
inline void solve() {
cin >> n >> s; pre();
vi r, g, y; for (int i = 0; i < n; i++) {
if (s[i] == 'R') r.pb(i);
else if (s[i] == 'G') g.pb(i);
else y.pb(i);
}
if (r.size()) dp[1][0][0][0] = r[0];
if (g.size()) dp[0][1][0][1] = g[0];
if (y.size()) dp[0][0][1][2] = y[0];
for (int rc = 0; rc <= r.size(); rc++) {
for (int gc = 0; gc <= g.size(); gc++) {
for (int yc = 0; yc <= y.size(); yc++) {
if (rc + gc + yc <= 1) continue;
bool d = false;
if (rc == 1 && gc == 0 && yc == 1) {
d = true;
}
// R is last
if (rc) {
int pos = r[rc - 1];
int mv = pos - min(gc, cnt(1, pos)) - min(yc, cnt(2, pos)) - (rc - 1);
// if (d) cout << "RLAST " << r[rc - 1] << " - " << (rc - 1) << " - " << cnt(g, pos) << " - " << cnt(y, pos) << '\n';
cmn(dp[rc][gc][yc][0], dp[rc - 1][gc][yc][1] + mv);
cmn(dp[rc][gc][yc][0], dp[rc - 1][gc][yc][2] + mv);
}
// G is last
if (gc) {
int pos = g[gc - 1];
int mv = pos - min(rc, cnt(0, pos)) - min(yc, cnt(2, pos)) - (gc - 1);
// if (d) cout << "GLAST " << mv << '\n';
cmn(dp[rc][gc][yc][1], dp[rc][gc - 1][yc][0] + mv);
cmn(dp[rc][gc][yc][1], dp[rc][gc - 1][yc][2] + mv);
}
// Y is last
if (yc) {
int pos = y[yc - 1];
int mv = pos - min(rc, cnt(0, pos)) - min(gc, cnt(1, pos)) - (yc - 1);
// if (d) cout << "YLAST " << y[yc - 1] << " - " << (yc - 1) << " - " << cnt(r, pos) << " - " << cnt(g, pos) << '\n';
cmn(dp[rc][gc][yc][2], dp[rc][gc][yc - 1][0] + mv);
cmn(dp[rc][gc][yc][2], dp[rc][gc][yc - 1][1] + mv);
}
}
}
}
// for (int i = 0; i < n; i++) {
// for (int j = 0; j < n; j++) {
// for (int k = 0; k < n; k++) {
// for (int l = 0; l < 3; l++) {
// cout << "DP " << i << " " << j << " " << k << " last " << "RGY"[l] << ": " << dp[i][j][k][l] << '\n';
// }
// }
// }
// }
int ans = min(dp[r.size()][g.size()][y.size()][0], min(dp[r.size()][g.size()][y.size()][1], dp[r.size()][g.size()][y.size()][2]));
cout << (ans == 1e18 ? -1 : ans) << '\n';
}
void setIO(string s) {
freopen((s + ".in").c_str(), "r", stdin);
freopen((s + ".out").c_str(), "w", stdout);
}
signed main() {
ios::sync_with_stdio(false);
cout.tie(0);
cin.tie(0);
//setIO();
int t = 1;
if (tc) {
cin >> t;
}
while (t--) {
solve();
}
}
