//#include"holiday.h"
#include <bits/stdc++.h>
#define ll long long
#define rep(i,m,n) for(int i=(m); i<=(n); i++)
#define reb(i,m,n) for(int i=(m); i>=(n); i--)
#define pii pair<int,int>
#define pll pair<ll,ll>
#define MP make_pair
#define fs first
#define se second
#define bit(msk, i) ((msk >> i) & 1)
#define iter(id, v) for(auto id : v)
#define pb push_back
#define SZ(v) (ll)v.size()
#define ALL(v) v.begin(),v.end()
using namespace std;
mt19937_64 rd(chrono :: steady_clock :: now ().time_since_epoch().count());
ll Rand (ll l, ll r) { return uniform_int_distribution<ll> (l, r) (rd); }
const int N = 4e2 + 7;
int Mod = 1e9 + 7;///lon
const int INF = 1e9;
const ll BASE = 137;
const int szBL = 450;
inline void minimize (int &A, int B) {
if (A > B) A = B;
}
int n;
int a[N];
vector<int> pos[5];
int szC[5];
int pre[5][N];
int dp[2][N][N][5]; ///0, 1
int cost (int cur, int p0, int p1, int p2) {
return max(0, pre[0][cur] - pre[0][p0]) + max(0, pre[1][cur] - pre[1][p1]) + max(0, pre[2][cur] - pre[2][p2]);
}
void solution () {
cin >> n;
string S;
cin >> S;
rep (i, 0, 2) pos[i].pb(0);
rep (i, 1, n) {
a[i] = (S[i - 1] == 'R' ? 0 : (S[i - 1] == 'G' ? 1 : 2));
pos[a[i]].pb(i);
szC[a[i]]++;
}
rep (j, 0, 2)
rep (i, 1, n) {
pre[j][i] = pre[j][i - 1] + (a[i] == j);
}
rep (j, 0, szC[0])
rep (k, 0, szC[1])
rep (t, 0, 2) dp[0][j][k][t] = INF;
dp[0][0][0][1] = 0;
dp[0][0][0][2] = 0;
dp[0][0][0][0] = 0;
rep (i, 0, n) {
rep (j, 0, szC[0])
rep (k, 0, szC[1])
rep (t, 0, 2) dp[i & 1 ^ 1][j][k][t] = INF;
rep (j, 0, szC[0])
rep (k, 0, szC[1]) {
int p = i - j - k;
// cout << i<<","<<j<<","<<k<<","<<p<<"\n";
if (j + k > i || p > szC[2])
continue;
rep (t, 0, 2) {
if (j < szC[0] && t != 0) {
minimize (dp[i & 1 ^ 1][j + 1][k][0], dp[i & 1][j][k][t] + cost (pos[0][j + 1], pos[0][j + 1], pos[1][k], pos[2][p]) );
}
if (k < szC[1] && t != 1) {
minimize (dp[i & 1 ^ 1][j][k + 1][1], dp[i & 1][j][k][t] + cost (pos[1][k + 1], pos[0][j], pos[1][k + 1], pos[2][p]) );
}
if (p < szC[2] && t != 2) {
minimize (dp[i & 1 ^ 1][j][k][2], dp[i & 1][j][k][t] + cost (pos[2][p + 1], pos[0][j], pos[1][k], pos[2][p + 1]) );
}
}
}
}
int res = INF;
rep (i, 0, 2) res = min(res, dp[n & 1][szC[0]][szC[1]][i]);
if (res > n * n) cout << -1;
else cout << res <<"\n";
}
#define file(name) freopen(name".inp","r",stdin); \
freopen(name".out","w",stdout);
int main () {
// file("c");
ios_base :: sync_with_stdio(false); cin.tie(0); cout.tie(0);
int num_Test = 1;
// cin >> num_Test;
while (num_Test--)
solution();
}
/*
no bug challenge +10
5 5
01101
10001
00110
10101
00100
*/
