#include <bits/stdc++.h>
using namespace std;
const long long INFll = (long long) 1e18 + 10;
const int INFii = (int) 1e9 + 10;
typedef long long ll;
typedef int ii;
typedef long double dbl;
#define endl '\n'
#define sc second
#define fr first
#define mp make_pair
#define pb push_back
#define all(x) x.begin(), x.end()
#define maxn 401
ii n, dp[maxn][maxn][maxn][4];
ii qtd1, qtd2, qtd3;
ii pos1[maxn], pos2[maxn], pos3[maxn];
ii cnt1[maxn], cnt2[maxn], cnt3[maxn];
string s;
int main() {
ios::sync_with_stdio(false); cin.tie(0);
//freopen("in.in", "r", stdin);
//freopen("out.out", "w", stdout);
cin >> n >> s;
for(ii i = 1; i <= n; i++) {
cnt1[i] = cnt1[i-1];
cnt2[i] = cnt2[i-1];
cnt3[i] = cnt3[i-1];
if(s[i-1] == 'R') {
cnt1[i]++;
pos1[++qtd1] = i;
}
if(s[i-1] == 'G') {
cnt2[i]++;
pos2[++qtd2] = i;
}
if(s[i-1] == 'Y') {
cnt3[i]++;
pos3[++qtd3] = i;
}
}
if(qtd1 > (n+1)/2 || qtd2 > (n+1)/2 || qtd3 > (n+1)/2) {
cout << -1 << endl;
return 0;
}
for(ii c1 = qtd1; c1 >= 0; c1--) {
for(ii c2 = qtd2; c2 >= 0; c2--) {
for(ii c3 = qtd3; c3 >= 0; c3--) {
for(ii ant = 1; ant <= 3; ant++) {
if(c1 == qtd1 && c2 == qtd2 && c3 == qtd3) {
dp[c1][c2][c3][ant] = 0;
continue;
}
ii ans = INFii;
if(c1 != qtd1 && ant != 1) {
ans = min(ans, dp[c1+1][c2][c3][1] + max(0,cnt2[pos2[c2]]-cnt2[pos1[c1+1]]) + max(0,cnt3[pos3[c3]]-cnt3[pos1[c1+1]]));
}
if(c2 != qtd2 && ant != 2) {
ans = min(ans, dp[c1][c2+1][c3][2] + max(0,cnt1[pos1[c1]]-cnt1[pos2[c2+1]]) + max(0,cnt3[pos3[c3]]-cnt3[pos2[c2+1]]));
}
if(c3 != qtd3 && ant != 3) {
ans = min(ans, dp[c1][c2][c3+1][3] + max(0,cnt2[pos2[c2]]-cnt2[pos3[c3+1]]) + max(0,cnt1[pos1[c1]]-cnt1[pos3[c3+1]]));
}
dp[c1][c2][c3][ant] = ans;
}
}
}
}
cout << min({dp[0][0][0][1],dp[0][0][0][2],dp[0][0][0][3]}) << endl;
}
