#include <bits/stdc++.h>
//#define int long long
#define P(x) do {if(debug) cout << x << endl;} while(false)
#define H(x) P(#x << ": " << x)
#define FR(i, a, b) for(int i = (a); i < (b); ++i)
#define F(i, n) FR(i, 0, n)
#define DR(i, a, b) for(int i = (b); i --> (a);)
#define D(i, n) DR(i, 0, n)
#define S(s) (int)(s).size()
#define ALL(x) (x).begin(), (x).end()
#define MI(x, a) (x) = min(x, a)
#define MA(x, a) (x) = max(x, a)
#define V vector
#define pb push_back
#define mp make_pair
using namespace std;
const bool debug = 0;
const int inf = 1e9;
int n;
vector<int> color;
vector<int> pos[3];
int dp[400][400][400][3];
bool vis[400][400][400][3];
vector<vector<int> > pref;
int offset(int a[3], int i){
int res = 0;
F(j,3){
res+=max(0,a[j] - pref[j][i]);
}
return res;
}
int solve(int a[3], int last){
int g = a[0], y = a[1], r = a[2];
H(g); H(y); H(r); H(last);
int j = g + y + r;
if(vis[g][y][r][last]){
P("visited, return: "<<dp[g][y][r][last]);
return dp[g][y][r][last];
}
if(g+y+r==n){
P("reached end");
return 0;
}
vis[g][y][r][last] = 1;
dp[g][y][r][last] = inf;
for(int i = 0; i<3; i++) if(i!=last && a[i] < pos[i].size()){
H(i);
H(offset(a,pos[i][a[i]]));
int extra = pos[i][a[i]] + offset(a,pos[i][a[i]]) - j;
a[i] ++;
dp[g][y][r][last] = min(dp[g][y][r][last], solve(a,i)+extra);
P("a");
a[i] --;
P("c");
}
P("d");
P(g<<" "<<y<<" "<<r<<" "<<last<<" returns "<< dp[g][y][r][last]);
return dp[g][y][r][last];
}
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
cin>>n;
map<char,int> mp;
color.resize(n);
mp['G'] = 0;
mp['Y'] = 1;
mp['R'] = 2;
pref.resize(3, vector<int> (n+1,0));
for(int i = 0; i<n; i++){
char c;
cin>>c;
color[i] = mp[c];
pos[mp[c]].push_back(i);
F(j,3){
pref[j][i+1] = pref[j][i];
}
pref[mp[c]][i+1]++;
}
int a[] = {0,0,0};
int ans = min(solve(a,0),solve(a,1));
if(ans < 1e6){
cout<<ans<<endl;
}
else{
cout<<-1<<endl;
}
return 0;
}
