#include <bits/stdc++.h>
using namespace std;
#define task "main"
#define F first
#define S second
#define ii pair<int, int>
#define il pair<int, long long>
#define li pair<long long, int>
#define FOR(i, a, b) for(int i = (a); i <= (b); ++i)
#define FOD(i, b, a) for(int i = (b); i >= (a); --i)
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 T>
void printArr(T container, string separator = " ", string finish = "\n", ostream &out = cout){
for(auto item: container) out << item << separator;
out << finish;
}
const int MAX_N = 403;
const int INF = (int)1e9;
int n;
string s;
int cnt[3][MAX_N];
vector<int> pos[3];
void solve() {
cin >> n >> s;
s = "#" + s;
FOR(i, 1, n) {
FOR(k, 0, 2) cnt[k][i] = cnt[k][i - 1];
int id = (s[i] == 'R' ? 0 : (s[i] == 'G' ? 1 : 2));
cnt[id][i]++;
pos[id].push_back(i);
}
int numR = (int)pos[0].size(), numG = (int)pos[1].size(), numY = (int)pos[2].size();
int dp[numR + 2][numG + 2][numY + 2][3];
FOR(r, 0, numR) FOR(g, 0, numG) FOR(y, 0, numY) FOR(col, 0, 2)
dp[r][g][y][col] = INF;
FOR(i, 0, 2) dp[0][0][0][i] = 0;
FOR(i, 0, numR) FOR(j, 0, numG) FOR(k, 0, numY) FOR(x, 0, 2) {
if (x != 0 && i < numR) {
int cost1 = max(0, cnt[1][pos[0][i]] - j);
int cost2 = max(0, cnt[2][pos[0][i]] - k);
minimize(dp[i + 1][j][k][0], dp[i][j][k][x] + cost1 + cost2);
}
if (x != 1 && j < numG) {
int cost0 = max(0, cnt[0][pos[1][j]] - i);
int cost2 = max(0, cnt[2][pos[1][j]] - k);
minimize(dp[i][j + 1][k][1], dp[i][j][k][x] + cost0 + cost2);
}
if (x != 2 && k < numY) {
int cost0 = max(0, cnt[0][pos[2][k]] - i);
int cost1 = max(0, cnt[1][pos[2][k]] - j);
minimize(dp[i][j][k + 1][2], dp[i][j][k][x] + cost0 + cost1);
}
}
int ans = INF;
FOR(x, 0, 2) minimize(ans, dp[numR][numG][numY][x]);
cout << (ans == INF ? -1 : ans) << "\n";
}
int32_t main() {
if (fopen(task".inp", "r")) {
freopen(task".inp", "r", stdin);
freopen(task".out", "w", stdout);
}
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
bool multitest = 0;
int numTest = 1;
if (multitest) cin >> numTest;
while (numTest--) {
solve();
}
return 0;
}
/* Lak lu theo dieu nhac!!!! */
