#include <bits/stdc++.h>
//#define int long long
//#pragma GCC optimize("Ofast")
//#pragma comment(linker, "/stack:200000000")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4")
#define file(s) freopen(s".in","r",stdin); freopen(s".out","w",stdout);
#define forev(i, b, a) for(int i = (b); i >= (a); --i)
#define forn(i, a, b) for(int i = (a); i <= (b); ++i)
#define all(x) x.begin(), x.end()
#define sz(s) (int)s.size()
#define pb push_back
#define ppb pop_back
#define mp make_pair
#define s second
#define f first
using namespace std;
typedef pair < long long, long long > pll;
typedef pair < int, int > pii;
typedef unsigned long long ull;
typedef vector < pii > vpii;
typedef vector < int > vi;
typedef long double ldb;
typedef long long ll;
typedef double db;
const int dx[] = {1, -1, 0, 0}, dy[] = {0, 0, 1, -1}, block = 555;
const int inf = 1e9, maxn = 4e5 + 148, mod = 1e9 + 7, N = 411;
const pii base = mp(1171, 3307), Mod = mp(1e9 + 7, 1e9 + 9);
const db eps = 1e-12, pi = 3.14159265359;
const ll INF = 1e18;
int n, cnt[N][3], dp[N][N][N][3];
vi pos[3];
string s;
int get (int l, int r, int i) {
if (l > r)
return 0;
return cnt[r][i] - cnt[l - 1][i];
}
int rec (vi c, int lst) {
// cerr << c[0] << ' ' << c[1] << ' ' << c[2] << ' ' << lst << endl;
if (c[0] + c[1] + c[2] == 0)
return 0;
if (!c[lst])
return inf;
int &res = dp[c[0]][c[1]][c[2]][lst];
if (res != -1)
return res;
c[lst]--;
res = inf;
int cost = 0;
for (int j = 0; j < 3; ++j)
if (j != lst && c[j])
cost += get(pos[lst][c[lst]], pos[j][c[j] - 1], j);
for (int j = 0; j < 3; ++j)
if (j != lst)
res = min(res, rec(c, j) + cost);
return res;
}
main () {
cin >> n >> s;
s = "#" + s;
memset(dp, -1, sizeof(dp));
for (int i = 1; i <= n; ++i) {
for (int j = 0; j < 3; ++j)
cnt[i][j] = cnt[i - 1][j];
if (s[i] == 'R') {
++cnt[i][0];
pos[0].pb(i);
} else if (s[i] == 'Y') {
++cnt[i][1];
pos[1].pb(i);
} else {
++cnt[i][2];
pos[2].pb(i);
}
}
vi c;
for (int j = 0; j < 3; ++j)
c.pb(cnt[n][j]);
int ans = min({rec(c, 0), rec(c, 1), rec(c, 2)});
if (ans == inf)
ans = -1;
cout << ans << endl;
}
