#include <bits/stdc++.h>
using namespace std;
#define all(x) x.begin(), x.end()
#define ar array
#define pb push_back
#define ln '\n'
//#define int long long
using i64 = long long;
template <class F, class _S>
bool chmin(F &u, const _S &v){
bool flag = false;
if ( u > v ){
u = v; flag |= true;
}
return flag;
}
template <class F, class _S>
bool chmax(F &u, const _S &v){
bool flag = false;
if ( u < v ){
u = v; flag |= true;
}
return flag;
}
const int inf = 1e9 + 1;
const int N = 201;
int dp[N * 2][N][N][4];
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int n; cin >> n;
string s; cin >> s;
s = '#' + s;
map <char,int> id;
id['R'] = 0, id['G'] = 1, id['Y'] = 2;
vector <ar<int,3>> pf(n + 1);
for ( int i = 1; i <= n; i++ ){
pf[i] = pf[i - 1];
pf[i][id[s[i]]]++;
}
{ // modifying
int it = 0;
for ( int j = 1; j <= 2; j++ ){
if ( pf[n][j] >= pf[n][it] ){
it = j;
}
}
for ( int j = 0; j <= n; j++ ){
swap(pf[j][it], pf[j][2]);
}
}
vector <vector<int>> pos(3, vector <int> (n + 2, -1));
for ( int i = 0; i < 3; i++ ){
for ( int j = 0; j <= n; j++ ){
int l = 0, r = n;
while ( l < r ){
int md = (l + r) >> 1;
if ( pf[md][i] < j ){
l = md + 1;
} else r = md;
}
if ( pf[l][i] >= j ){
pos[i][j] = l;
}
}
}
for ( int i = 0; i <= n; i++ ){
for ( int j = 0; j <= pf[n][0]; j++ ){
for ( int k = 0; k <= pf[n][1]; k++ ){
for ( int lst = 0; lst <= 3; lst++ ){
dp[i][j][k][lst] = inf;
}
}
}
}
dp[0][0][0][3] = 0;
for ( int i = 0; i < n; i++ ){
for ( int j = 0; j <= pf[n][0]; j++ ){
for ( int k = 0; k <= pf[n][1]; k++ ){
for ( int lst = 0; lst < 3 + !i; lst++ ){
int &tmp = dp[i][j][k][lst];
if ( tmp == inf ){
continue;
}
int p = i - (j + k);
vector <int> a(3), t = {j, k, p};
for ( int it = 0; it < 3; it++ ){
int l = pos[it][t[it] + 1];
if ( l == -1 ){
a[it] = -1;
} else{
for ( int q = 0; q < 3; q++ ){
if ( q != it ){
a[it] += max(0, pf[l][q] - t[q]);
}
}
}
}
if ( a[0] != -1 && lst != 0 ){
chmin(dp[i + 1][j + 1][k][0], tmp + a[0]);
}
if ( a[1] != -1 && lst != 1 ){
chmin(dp[i + 1][j][k + 1][1], tmp + a[1]);
}
if ( a[2] != -1 && lst != 2 ){
chmin(dp[i + 1][j][k][2], tmp + a[2]);
}
}
}
}
}
int opt = inf;
for ( int i = 0; i < 3; i++ ){
chmin(opt, dp[n][pf[n][0]][pf[n][1]][i]);
}
if ( opt == inf ){
opt = -1;
}
cout << opt;
cout << '\n';
}
