#include <bits/stdc++.h>
using namespace std;
#define mod 1000000007
#define mp make_pair
#define pb push_back
#define bas(x) #x << ": " << x << " "
#define prarr(x, n) cout << #x << ": "; for (int qsd = 0; qsd < n; qsd++) cout << x[qsd] << " "; cout << endl;
#define prarrv(x) cout << #x << ": "; for (int qsd = 0; qsd < (int)x.size(); qsd++) cout << x[qsd] << " "; cout << endl;
#define ppair(x) "(" << x.first << ", " << x.second << ")"
#define inside sl<=l&&r<=sr
#define outside sr<l||r<sl
typedef long long ll;
string s;
int dist(string str){
string ana = s;
ll ans = 0;
int say = 0;
for (int i = str.size()-1; i >= 0; i--){
for (int j = ana.size()-1; j >= 0; j--){
if (ana[j] == str[i]){
ana[j] = 'x';
say++;
break;
} else if (ana[j] != 'x') ans++;
}
}
if (say != str.size()) return 1000000000;
return ans;
}
string dp[65][65][65][3];
string f(int r, int g, int y, int last){
//cout << "f(" << r << ", " << g << ", " << y << ", " << last << ")" << endl;
if (dp[r][g][y][last].size() && dp[r][g][y][last][0] != '-') return dp[r][g][y][last];
string ans = "-";
int minn = 1000000000;
if (r > 0 && last != 0){
string ret = f(r-1, g, y, 0);
//if (ret.size() && ret[0] == '-');
//else {
ret = "R" + ret;
int d = dist(ret);
if (minn > d){
minn = d;
ans = ret;
}
//}
}
if (g > 0 && last != 1){
string ret = f(r, g-1, y, 1);
ret = "G" + ret;
int d = dist(ret);
if (minn > d){
minn = d;
ans = ret;
}
}
if (y > 0 && last != 2){
string ret = f(r, g, y-1, 2);
ret = "Y" + ret;
int d = dist(ret);
if (minn > d){
minn = d;
ans = ret;
}
}
if (g == 0 && r == 0 && y == 0){
return dp[r][g][y][last] = "";
}
return dp[r][g][y][last] = ans;
}
int main(){
//freopen("in", "r", stdin);
//freopen("out", "w", stdout);
int n;
cin >> n;
cin >> s;
for (int i1 = 0; i1 < 65; i1++)
for (int i2 = 0; i2 < 65; i2++)
for (int i3 = 0; i3 < 65; i3++)
for (int i4 = 0; i4 < 3; i4++)
dp[i1][i2][i3][i4] = "-";
int r = 0, g = 0, y = 0;
for (int i = 0; i < n; i++){
r += s[i] == 'R';
g += s[i] == 'G';
y += s[i] == 'Y';
}
string ans = f(r, g, y, -1);
//cout << ans << endl;
ll sonuc = dist(ans);
if (sonuc == 1000000000) cout << -1 << endl;
else cout << sonuc << endl;
}
