#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))
#define FIN ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
using namespace std;
typedef long long ll;
string opc = "RGY";
const int MAXN = 400+1;
int dp[401][5][202][202];
ll n;
string s;
vector<vector<ll>> pos(3);
map<char,ll> ri;
ll select(ll k , ll R, ll G){
if(k==0) return R;
if(k==1) return G;
}
int f(ll x, ll ant, ll R, ll G){
int &res = dp[x][ant][R][G];
if(res!=-1) return res;
if(x==n) return 0;
res = 10000000;
forn(k,0,2){
if(k==ant) continue;
ll rgy = select(k,R,G);
if(rgy==SZ(pos[k])) continue;
int cost = 0;
ll j = pos[k][rgy];
forn(h,0,3){
ll mrgy = select(h,R,G);
cost += max((ll)0,(lower_bound(ALL(pos[h]),j)-pos[h].begin()) - mrgy);
}
ll nR = R; if(k==0) nR++;
ll nG = G; if(k==1) nG++;
res=min(res, f(x+1,k,nR,nG)+cost);
}
//cout<<x<<" "<<R<<" "<<G<<" "<<Y<<"-------> "<<res<<'\n';
return res;
}
int main(){ FIN;
cin>>n;
cin>>s;
ri['R']=0;
ri['G']=1;
forn(i,0,n) pos[ri[s[i]]].pb(i);
if(SZ(pos[0])>=202 || SZ(pos[1])>=202 || abs(SZ(pos[0])-SZ(pos[1]))>=2){
cout<<-1<<'\n';
exit(0);
}
mset(dp,-1);
ll res=f(0,4,0,0);
if(res>1000000000) cout<<"-1"<<'\n';
else cout<<res<<'\n';
return 0;
}
