#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
typedef long long ll;
#define st first
#define nd second
#define f(a, c, b) for (int a = c; b > a; a++)
#define pb push_back
#define all(a) a.begin(), a.end()
#define sz(a) int(a.size())
const int DUZO = 1'000'000;
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
int n;
cin >> n;
vector<int> a(n);
f(i, 0, n) {
char p1;
cin >> p1;
if (p1 == 'R') a[i] = 0;
else if (p1 == 'Y') a[i] = 1;
else a[i] = 2;
}
vector<vector<int>> wstp = {{-1}, {-1}, {-1}};
f(i, 0, n) wstp[a[i]].pb(i);
if ((max(sz(wstp[0]), max(sz(wstp[1]), sz(wstp[2]))) - 1) > (n + 1)/2) {
cout << -1;
return 0;
}
vector<vector<vector<int>>> ile(3); //ile[a][b][c] - ile z b pierwszych elementow z grupy a jest wieksze niz c;
f(i, 0, 3) {
ile[i].resize(sz(wstp[i]) + 1, vector<int>(n));
}
f(i, 0, 3) ile[i][0][0] = 0;
f(grupa, 0, 3) {
f(pref, 1, sz(wstp[grupa])) {
f(val, 0, n) {
ile[grupa][pref][val] = ile[grupa][pref - 1][val];
if (wstp[grupa][pref] > val) ile[grupa][pref][val]++;
}
}
}
vector<vector<vector<vector<int>>>> dp(sz(wstp[0]), vector<vector<vector<int>>>(sz(wstp[1]), vector<vector<int>>(sz(wstp[2]), vector<int>(3)))); //dp[a][b][c][d] - jaki jest wynik dla odpowiednich prefikosw wst - d to ostatni element
f(i, 0, 3) dp[0][0][0][i] = 0;
f(a, 0, sz(wstp[0])) {
f(b, 0, sz(wstp[1])) {
f(c, 0, sz(wstp[2])) {
if (a == b && b == c && c == 0) continue;
f(ost, 0, 3) {
vector<int> ile_aktl_wstp = {a, b, c};
if (ile_aktl_wstp[ost] == 0 || (((ile_aktl_wstp[0] + ile_aktl_wstp[1] + ile_aktl_wstp[2]) + 1)/2) < max(ile_aktl_wstp[0], max(ile_aktl_wstp[1], ile_aktl_wstp[2]))) {
dp[a][b][c][ost] = DUZO;
continue;
}
int koszt = wstp[ost][ile_aktl_wstp[ost]];
f(i, 0, 3) {
if (i == ost) continue;
koszt += ile[i][ile_aktl_wstp[i]][wstp[ost][ile_aktl_wstp[ost]]];
}
koszt -= (a + b + c - 1);
if (ost == 0) {
dp[a][b][c][ost] = min(dp[a - 1][b][c][1], dp[a - 1][b][c][2]);
} else if (ost == 1) {
dp[a][b][c][ost] = min(dp[a][b - 1][c][0], dp[a][b - 1][c][2]);
} else {
dp[a][b][c][ost] = min(dp[a][b][c - 1][0], dp[a][b][c - 1][1]);
}
dp[a][b][c][ost] += koszt;
}
}
}
}
int wnk = DUZO;
f(i, 0, 3) {
wnk = min(wnk, dp[sz(wstp[0]) - 1][sz(wstp[1]) - 1][sz(wstp[2]) - 1][i]);
}
cout << wnk;
/*f(a, 0, sz(wstp[0])) {
f(b, 0, sz(wstp[1])) {
f(c, 0, sz(wstp[2])) {
f(ost, 0, 3) {
cout << "dp[" << a << "][" << b << "][" << c << "][" << ost << "] = " << dp[a][b][c][ost] << "\n";
}
}
}
}*/
return 0;
}
