#include <bits/stdc++.h>
#define rep(a,b,c) for(auto a = (b); a != (c); a++)
#define repD(a,b,c) for(auto a = (b); a != (c); a--)
#define repIn(a, b) for(auto& a : (b))
#define repIn2(a, b, c) for(auto& [a, b] : (c))
constexpr bool dbg = 1;
#define DEBUG if constexpr(dbg)
#define DC DEBUG std::cerr
#define eol std::endl
#define ll long long
#define pb push_back
using namespace std;
vector<char> V = {'Y', 'R', 'G'};
map<char, int> M = { {'Y', 0}, {'R', 1}, {'G', 2} };
int n;
string s, s2;
int ans = 1e9 + 9;
void brutas(int ind) {
if(ind == n) {
vector<int> cnt(3, 0);
repIn(i, s) cnt[M[i]]++;
repIn(i, s2) cnt[M[i]]--;
repIn(i, cnt) if(i != 0) return;
int cost = 0;
string sc = s;
repIn(i, s2) {
auto it = sc.begin();
while(*it != i) it++, cost++;
sc.erase(it);
}
ans = min(ans, cost);
return;
}
rep(i, 0, 3) {
if(ind && s2[ind - 1] == V[i]) continue;
s2[ind] = V[i];
brutas(ind + 1);
}
}
int main() {
ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
cin >> n >> s;
s2.resize(n);
brutas(0);
cout << (ans == (int)1e9 + 9 ? -1 : ans) << eol;
}
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