#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using ull = unsigned long long;
#define all(x) x.begin(), x.end()
template <typename T1, typename T2> inline void chkmin(T1 &x, const T2 & y) {if (x > y) x = y;}
template <typename T1, typename T2> inline void chkmax(T1 &x, const T2 & y) {if (x < y) x = y;}
const int MAXN = 410;
int n;
int a[MAXN];
int pos[MAXN][3];
int cnt[MAXN][3];
void read() {
cin >> n;
for (int i = 1; i <= n; i++) {
char x;
cin >> x;
if (x == 'R') a[i] = 0;
else if (x == 'G') a[i] = 1;
else a[i] = 2;
for (int j = 0; j < 3; j++)
cnt[i][j] = cnt[i - 1][j];
cnt[i][a[i]]++;
pos[cnt[i][a[i]]][a[i]] = i;
}
}
const int INF = 1e9 + 10;
vector<vector<vector<vector<int>>>> dp;
void build() {
dp.assign(cnt[n][0] + 1, vector<vector<vector<int>>> (cnt[n][1] + 1, vector<vector<int>> (cnt[n][2] + 1, vector<int> (3, INF))));
dp[0][0][0][0] = dp[0][0][0][1] = dp[0][0][0][2] = 0;
}
void calc() {
for (int i = 0; i <= cnt[n][0]; i++) {
for (int j = 0; j <= cnt[n][1]; j++) {
for (int k = 0; k <= cnt[n][2]; k++) {
for (int ch = 0; ch < 3; ch++) {
if (ch != 0 && i + 1 <= cnt[n][0]) {
int p = pos[i + 1][0];
chkmin(dp[i + 1][j][k][0], dp[i][j][k][ch] + max(0, cnt[p][1] - j) + max(0, cnt[p][2] - k));
}
if (ch != 1 && j + 1 <= cnt[n][1]) {
int p = pos[j + 1][1];
chkmin(dp[i][j + 1][k][1], dp[i][j][k][ch] + max(0, cnt[p][0] - i) + max(0, cnt[p][2] - k));
}
if (ch != 2 && k + 1 <= cnt[n][2]) {
int p = pos[k + 1][2];
chkmin(dp[i][j][k + 1][2], dp[i][j][k][ch] + max(0, cnt[p][0] - i) + max(0, cnt[p][1] - j));
}
}
}
}
}
}
int ans;
void make_ans() {
ans = INF;
for (int i = 0; i < 3; i++) {
chkmin(ans, dp[cnt[n][0]][cnt[n][1]][cnt[n][2]][i]);
}
if (ans == INF) ans = -1;
}
void run() {
build();
calc();
make_ans();
}
void write() {
cout << ans << endl;
}
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
read();
run();
write();
return 0;
}
