#include <bits/stdc++.h>
using namespace std;
// #define int long long
#define ull unsigned long long
#define ld long double
#define pii pair<int, int>
#define fi first
#define se second
#define __builtin_popcount __builtin_popcountll
#define all(x) (x).begin(), (x).end()
#define BIT(x, i) (((x) >> (i)) & 1)
#define MASK(x) ((1ll << (x)))
#define debug(a, l, r) for (int _i = (l); _i <= (r); ++_i) cout << (a)[_i] << ' '; cout << '\n';
const int MAXN = 405;
const int INF = 1e9 + 7;
int n, cnt[MAXN], pos[3][MAXN], pref[3][MAXN];
int dp[3][MAXN][MAXN][MAXN];
string s;
void init() {
cin >> n >> s;
s = ' ' + s;
for (int i = 1; i <= n; ++i) {
for (int j = 0; j <= 2; ++j) pref[j][i] = pref[j][i - 1];
char c = s[i];
if (c == 'R') {
cnt[0]++;
pos[0][cnt[0]] = i;
pref[0][i]++;
} else if (c == 'G') {
cnt[1]++;
pos[1][cnt[1]] = i;
pref[1][i]++;
} else {
cnt[2]++;
pos[2][cnt[2]] = i;
pref[2][i]++;
}
}
}
void solve() {
for (int i = 0; i <= 2; ++i)
for (int r = 0; r <= cnt[0]; ++r)
for (int g = 0; g <= cnt[1]; ++g)
for (int y = 0; y <= cnt[2]; ++y) {
if (!r && !g && !y) dp[i][r][g][y] = 0;
else dp[i][r][g][y] = INF;
}
for (int r = 0; r <= cnt[0]; ++r) {
for (int g = 0; g <= cnt[1]; ++g) {
for (int y = 0; y <= cnt[2]; ++y) {
for (int cur = 0; cur <= 2; ++cur) {
for (int nxt = 0; nxt <= 2; ++nxt) {
if (cur == nxt) continue;
if (nxt == 0 && r == cnt[0]) continue;
if (nxt == 1 && g == cnt[1]) continue;
if (nxt == 2 && y == cnt[2]) continue;
// cout << "State " << cur << " (" << r << ", " << g << ", " << y << ") -> ";
// cout << nxt;
// cout << '\n';
int nr = r, ng = g, ny = y;
int cost = 0;
if (nxt == 0) {
nr = r + 1;
int p = pos[0][r + 1];
// for (int i = 1; i <= ng; ++i)
// if (pos[1][i] > p) ++cost;
// for (int i = 1; i <= ny; ++i)
// if (pos[2][i] > p) ++cost;
cost += max(0, pref[1][pos[1][ng]] - pref[1][p]);
cost += max(0, pref[2][pos[2][ny]] - pref[2][p]);
} else if (nxt == 1) {
ng = g + 1;
int p = pos[1][g + 1];
// for (int i = 1; i <= nr; ++i)
// if (pos[0][i] > p) ++cost;
// for (int i = 1; i <= ny; ++i)
// if (pos[2][i] > p) ++cost;
cost += max(0, pref[0][pos[0][nr]] - pref[0][p]);
cost += max(0, pref[2][pos[2][ny]] - pref[2][p]);
} else {
ny = y + 1;
int p = pos[2][y + 1];
// for (int i = 1; i <= nr; ++i)
// if (pos[0][i] > p) ++cost;
// for (int i = 1; i <= ng; ++i)
// if (pos[1][i] > p) ++cost;
cost += max(0, pref[0][pos[0][nr]] - pref[0][p]);
cost += max(0, pref[1][pos[1][ng]] - pref[1][p]);
}
dp[nxt][nr][ng][ny] = min(dp[nxt][nr][ng][ny], dp[cur][r][g][y] + cost);
}
}
}
}
}
int res = INF;
for (int i = 0; i <= 2; ++i)
res = min(res, dp[i][cnt[0]][cnt[1]][cnt[2]]);
cout << (res == INF ? -1 : res);
}
signed main() {
#ifdef NCTHANH
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
ios_base::sync_with_stdio(0);
cin.tie(nullptr); cout.tie(nullptr);
init();
solve();
return 0;
}
