#include <bits/stdc++.h>
using namespace std;
// #define int long long
#define endl "\n"
#define pb push_back
#define ff first
#define ss second
#define ii pair<int, int>
#define vi vector<int>
#define vii vector<pair<int, int>>
#define yes cout << "YES\n"
#define no cout << "NO\n"
#define mii map<int, int>
#define rep(i, a, b) for (int i = a; i <= b; i++)
#define all(a, len) (a) + 1, (a) + len + 1
#define vall(a) (a).begin(), a.end()
const int INF = 5e8;
const int MOD = 1e9 + 7;
int n; const int mx = 405;
string s;
int dp[mx][mx][mx][3];
int idx[mx][3], nR, nG, nY;
int p[mx][3];
void CONQUEROR() {
cin >> n >> s;
s = "#" + s;
for (char c: {'R', 'Y', 'G'}) {
if (count(s.begin() + 1, s.end(), c) > (n+1)/2 && n > 1) {
cout << -1;
exit(0);
}
}
}
void preprocess() {
nR = nG = nY = 0;
rep(i, 1, n) {
rep(j, 0, 2) {
p[i][j] = (p[i-1][j]);
}
if (s[i] == 'R') {
idx[++nR][0] = i;
p[i][0]++;
} else if (s[i] == 'G') {
idx[++nG][1] = i;
p[i][1]++;
} else {
idx[++nY][2] = i;
p[i][2]++;
}
}
}
inline int cost(int i, int j1, int j2, int col) {
int u = i - j1 - j2;
if (col == 0) {
if (j1 >= nR) return INF;
int pos = idx[j1 + 1][0];
int them = 0;
if (p[pos][1] < j2) them += j2 - p[pos][1];
if (p[pos][2] < u) them += u - p[pos][2];
return abs(i + 1 - (pos + them));
} else if (col == 1) {
if (j2 >= nG) return INF;
int pos = idx[j2 + 1][1];
int them = 0;
if (p[pos][0] < j1) them += j1 - p[pos][0];
if (p[pos][2] < u) them += u - p[pos][2];
return abs(i + 1 - (pos + them));
} else {
if (u >= nY) return INF;
int pos = idx[u + 1][2];
int them = 0;
if (p[pos][0] < j1) them += j1 - p[pos][0];
if (p[pos][1] < j2) them += j2 - p[pos][1];
return abs(i + 1 - (pos + them));
}
}
inline void mini(int& x, int y) {
if (y < x) x = y;
}
void OF() {
preprocess();
fill(&dp[0][0][0][0], &dp[0][0][0][0] + mx * mx * mx * 3, INF);
dp[0][0][0][0] = 0;
dp[0][0][0][1] = 0;
dp[0][0][0][2] = 0;
for (int i = 0; i < n; i++) {
for (int j1 = 0; j1 <= nR; j1++) {
for (int j2 = 0; j2 <= nG; j2++) {
if (i - j1 - j2 > nY) continue;
for (int t = 0; t <= 2; t++) {
if (dp[i][j1][j2][t] == INF) continue;
for (int t_ = 0; t_ <= 2; t_++) if (t_ != t) {
mini(dp[i + 1][j1 + (t_ == 0)][j2 + (t_ == 1)][t_],
dp[i][j1][j2][t] + cost(i, j1, j2, t_));
}
}
}
}
}
// cerr << dp[1][1][0][0] << endl;
// cerr << cost(1, 1, 0, 2) << endl;
// cerr << dp[2][1][0][2] << endl;
int ans = INF;
rep(t, 0, 2) ans = min(ans, dp[n][nR][nG][t]);
cout << ans;
}
void DPMQ() {
}
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr); cout.tie(nullptr);
int TEST = 1;
// cin >> TEST;
while (TEST--) {
CONQUEROR();
OF();
DPMQ();
}
return 0;
}
