#include<bits/stdc++.h>
#define MOD 1000000007
#define all(x) x.begin(),x.end()
#define ff first
#define ss second
#define pb push_back
#define fast ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
using namespace std;
int pre[405][2],pre2[405][2],pre3[405][2];
vector<int>a,b,c;
vector<vector<vector<vector<int>>>>dp;
inline int f(int x,int y,int z,int t){
if(x==a.size() && y==b.size() && z==c.size())return 0;
if(dp[x][y][z][t]!=-1)return dp[x][y][z][t];
int mn=1e9;
if(x!=a.size() && t!=1){
int ind=pre[x][0];
ind=min(ind,y);
int ind2=pre[x][1];
ind2=min(ind2,z);
mn=min(mn,f(x+1,y,z,1)+y-ind+z-ind2);
}
if(y!=b.size() && t!=2){
int ind=pre2[y][0];
ind=min(ind,x);
int ind2=pre2[y][1];
ind2=min(ind2,z);
mn=min(mn,f(x,y+1,z,2)+x-ind+z-ind2);
}
if(z!=c.size() && t!=3){
int ind=pre3[z][0];
ind=min(ind,x);
int ind2=pre3[z][1];
ind2=min(ind2,y);
mn=min(mn,f(x,y,z+1,3)+x-ind+y-ind2);
}
return dp[x][y][z][t]=mn;
}
int32_t main(){
fast;
int n;
cin>>n;
string s;
cin>>s;
for(int i=0;i<n;i++){
if(s[i]=='R'){
a.pb(i);
}
if(s[i]=='G'){
b.pb(i);
}
if(s[i]=='Y'){
c.pb(i);
}
}
for(int i=0;i<a.size();i++){
pre[i][0]=upper_bound(all(b),a[i])-b.begin();
pre[i][1]=upper_bound(all(c),a[i])-c.begin();
}
for(int i=0;i<b.size();i++){
pre2[i][0]=upper_bound(all(a),b[i])-a.begin();
pre2[i][1]=upper_bound(all(c),b[i])-c.begin();
}
for(int i=0;i<c.size();i++){
pre3[i][0]=upper_bound(all(a),c[i])-a.begin();
pre3[i][1]=upper_bound(all(b),c[i])-b.begin();
}
dp.assign((int)a.size()+5,vector<vector<vector<int>>>((int)b.size()+5,vector<vector<int>>((int)c.size()+5,vector<int>(4,-1))));
int ans=f(0,0,0,0);
if(ans==1e9)cout<<-1<<endl;
else cout<<ans<<endl;
}
