#include <iostream>
#include<vector>
#define int long long
using namespace std;
string alt(int n, bool c) {
string ans = "";
while (n--)
ans += "RG"[c], c ^= 1;
return ans;
}
int cnt(string s, string t) { //s->t
int n = s.length(), ans = 0;
for (int i = 0; i < n; ++i)
if (s[i] != t[i]) {
int val = 0;
for (int j = i + 1; j < n; ++j)
if (s[j] == t[i]) {
val = j; break;
}
for (int j = val; j > i; --j)
swap(s[j], s[j - 1]), ans++;
}
return ans;
}
int res = 1e17, gc = 0, rc = 0, yc = 0;
string s;
void sol(char p, string& c, int n) {
if (c.length() == n) {
int r = 0, y = 0, g = 0;
for (int i = 0; i < n; ++i)
if (c[i] == 'R') r++;
else if (c[i] == 'G') g++;
else y++;
if ((r != rc) || (y != yc) || (g != gc))
return;
res = min(res, cnt(s, c));
return;
}
for (int i = 0; i < 3; ++i)
if ("RGY"[i] != p) {
c += "RGY"[i];
sol("RGY"[i], c, n);
c.pop_back();
}
}
int32_t main()
{
int n, r = 0, g = 0; cin >> n;
string a1 = alt(n, 0), a2 = alt(n, 1); cin >> s;
for (int i = 0; i < n; ++i)
if (s[i] == 'R') r++;
else if (s[i] == 'G') g++;
else yc++;
gc = g, rc = r;
if (n <= 20) {
string c = "";
for (int i = 0; i < 3; ++i) {
c = "RGY"[i]; sol("RGY"[i], c, n);
}
cout << (res == 1e17 ? -1 : res);
return 0;
}
if (abs(g - r) > 1) { cout << -1; return 0; }
int ans = 1e17;
if (g >= r) ans = min(ans, cnt(s, a2));
if (r >= g) ans = min(ans, cnt(s, a1));
cout << ans;
}
