#include<bits/stdc++.h>
using namespace std;
using LL=long long;
#define FOR(i,l,r) for(int i=(l);i<=(r);++i)
#define REP(i,n) FOR(i,0,(n)-1)
#define ssize(x) int(x.size())
template<class A,class B>auto&operator<<(ostream&o,pair<A,B>p){return o<<'('<<p.first<<", "<<p.second<<')';}
template<class T>auto operator<<(ostream&o,T x)->decltype(x.end(),o){o<<'{';int i=0;for(auto e:x)o<<(", ")+2*!i++<<e;return o<<'}';}
#ifdef DEBUG
#define debug(x...) cerr<<"["#x"]: ",[](auto...$){((cerr<<$<<"; "),...)<<'\n';}(x)
#else
#define debug(...) {}
#endif
int main() {
cin.tie(0)->sync_with_stdio(0);
const int INF = 1e9;
int n;
cin >> n;
map<char, int> s;
s['R'] = 0;
s['G'] = 1;
s['Y'] = 2;
vector<vector<int>> t(3, vector<int> (1, 0)), sum(3, vector<int> (n + 1, 0));
vector<int> v(n);
FOR (i, 1, n) {
char c;
cin >> c;
t[s[c]].emplace_back(i);
sum[s[c]][i] = 1;
}
FOR (i, 1, n) {
REP (j, 3) {
sum[j][i] += sum[j][i - 1];
}
}
vector<vector<vector<array<int, 3>>>> dp(sum[0][n] + 1, vector<vector<array<int, 3>>> (sum[1][n] + 1, vector<array<int, 3>> (sum[2][n] + 1, array<int, 3> {INF, INF, INF})));
REP (i, 3) {
if (sum[i][n] > (n + 1) / 2) {
cout << "-1" << endl;
return 0;
}
dp[0][0][0][i] = 0;
}
FOR (i, 0, sum[0][n]) {
FOR (j, 0, sum[1][n]) {
FOR (k, 0, sum[2][n]) {
if (i) {
dp[i][j][k][0] = min(dp[i - 1][j][k][1], dp[i - 1][j][k][2]) +
max(0, sum[1][t[0][i]] - sum[1][t[1][j]]) +
max(0, sum[2][t[0][i]] - sum[2][t[2][k]]);
}
if (j) {
dp[i][j][k][1] = min(dp[i][j - 1][k][0], dp[i][j - 1][k][2]) +
max(0, sum[0][t[1][j]] - sum[0][t[0][i]]) +
max(0, sum[2][t[1][j]] - sum[2][t[2][k]]);
}
if (k) {
dp[i][j][k][2] = min(dp[i][j][k - 1][0], dp[i][j][k - 1][1]) +
max(0, sum[0][t[2][k]] - sum[0][t[0][i]]) +
max(0, sum[1][t[2][k]] - sum[1][t[1][j]]);
}
}
}
}
int odp = INF;
REP (i, 3) {
odp = min(odp, dp[sum[0][n]][sum[1][n]][sum[2][n]][i]);
}
if (odp == INF) {
cout << "-1" << endl;
return 0;
}
cout << odp << endl;
}
