#include <bits/stdc++.h>
#define fst first
#define snd second
#define pb push_back
#define forn(i,a,b) for(int i = a; i<b; i++)
#define SZ(x) (int)x.size()
#define ALL(x) x.begin(),x.end()
#define mset(a,v) memset(a,v,sizeof(a))
using namespace std;
typedef long long ll;
map<char,vector<ll>> pos;
ll nextp(char c, ll i){
ll res = 1000000;
if(c!='G'){
ll ig = lower_bound(ALL(pos['G']),i)-pos['G'].begin();
if(ig<SZ(pos['G'])) res=min(res,pos['G'][ig]);
}
if(c!='Y'){
ll iy = lower_bound(ALL(pos['Y']),i)-pos['Y'].begin();
if(iy<SZ(pos['Y'])) res=min(res,pos['Y'][iy]);
}
if(c!='R'){
ll ir = lower_bound(ALL(pos['R']),i)-pos['R'].begin();
if(ir<SZ(pos['R'])) res=min(res,pos['R'][ir]);
}
return res;
}
int main(){
ll n; cin>>n;
string s; cin>>s;
forn(i,0,n) pos[s[i]].pb(i);
pos['R'].pb(1000000);
pos['G'].pb(1000000);
pos['Y'].pb(1000000);
ll res = 0;
forn(i,0,n-1){
if(s[i]==s[i+1]){
ll j = nextp(s[i],i+1);
// cout<<i<<" "<<i+1<<" "<<j<<'\n';
ll oj = j;
if(j>=n) continue;
for(j=oj; j>i+1; j--){
res++;
char aux = s[j-1];
s[j-1]=s[j];
s[j]=aux;
}
pos[s[i+1]].erase(lower_bound(ALL(pos[s[i+1]]),oj));
pos[s[i+1]].pb(i+1);
sort(ALL(pos[s[i+1]]));
//cout<<s<<'\n';
}
}
//cout<<" RES: ";
// cout<<s<<'\n';
reverse(ALL(s));
pos['R'].clear();
pos['G'].clear();
pos['Y'].clear();
forn(i,0,n) pos[s[i]].pb(i);
pos['R'].pb(1000000);
pos['G'].pb(1000000);
pos['Y'].pb(1000000);
forn(i,0,n-1){
if(s[i]==s[i+1]){
ll j = nextp(s[i],i+1);
ll oj = j;
if(j>=n) continue;
for(j=oj; j>i+1; j--){
res++;
char aux = s[j-1];
s[j-1]=s[j];
s[j]=aux;
}
pos[s[i+1]].erase(pos[s[i+1]].begin()+oj);
pos[s[i+1]].pb(i+1);
sort(ALL(pos[s[i+1]]));
}
}
// cout<<s<<'\n';
bool yes=true;
forn(i,0,n-1){
if(s[i]==s[i+1]) yes=false;
}
if(yes) cout<<res<<'\n';
else cout<<-1<<'\n';
return 0;
}
