#define here cerr<<"===========================================\n"
#define dbg(x) cerr<<#x<<": "<<x<<endl;
#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
#define ld double
#define ll int
#define llinf 1000000000 // 10^17
#define pb push_back
#define popb pop_back
#define fi first
#define sc second
#define endl '\n'
#define pll pair<ll,ll>
#define pld pair<ld,ld>
#define sz(a) (ll)(a.size())
#define all(a) a.begin(),a.end()
#define ceri(a,l,r) {cerr<<#a<<": ";for(ll i_ = l;i_<=r;i_++) cerr<<a[i_]<< " ";cerr<<endl;}
#define daj_mi_malo_vremena ios_base::sync_with_stdio(false);cerr.tie(0);cout.tie(0);cin.tie(0);
using namespace std;
using namespace __gnu_pbds;
/*
ll add(ll x,ll y){
x+=y;
if(x<0){
x%=mod;
x+=mod;
}else{
if(x>=mod) x%=mod;
}
return x;
}
ll mul(ll a,ll b){
ll ans = (a*b)%mod;
if(ans<0) ans+=mod;
return ans;
}
typedef tree<int,null_type,less<ll>,rb_tree_tag,tree_order_statistics_node_update> ordered_set;
typedef tree<int,null_type,less_equal<ll>,rb_tree_tag,tree_order_statistics_node_update> ordered_multiset;
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
ll rnd(ll l,ll r){
return uniform_int_distribution<ll>(l,r)(rng);
}
*/
#define maxn 405
ll n;
ll a[maxn];
ll dp[maxn][maxn][maxn][4];
bool bio[maxn][maxn][maxn][4];
ll sz[4];
ll pos[4][maxn];
ll calc[4][4][maxn][maxn];
ll reshi(ll r,ll g,ll y,ll last){
if(r+g+y==0) return 0;
if(bio[r][g][y][last]) return dp[r][g][y][last];
ll &ans = dp[r][g][y][last];
bio[r][g][y][last] = 1;
ans = llinf;
if(r&&last!=1) ans = min(ans,reshi(r-1,g,y,1)+calc[1][2][r][g]+calc[1][3][r][y]);
if(g&&last!=2) ans = min(ans,reshi(r,g-1,y,2)+calc[2][1][g][r]+calc[2][3][g][y]);
if(y&&last!=3) ans = min(ans,reshi(r,g,y-1,3)+calc[3][1][y][r]+calc[3][2][y][g]);
return ans;
}
int main(){
daj_mi_malo_vremena
cin >> n; string s; cin >> s;
for(ll i = 1;i<=n;i++){
if(s[i-1]=='R') pos[1][++sz[1]] = i;
if(s[i-1]=='G') pos[2][++sz[2]] = i;
if(s[i-1]=='Y') pos[3][++sz[3]] = i;
}
for(ll e = 1;e<=3;e++){
for(ll f = 1;f<=3;f++){
if(e==f) continue;
for(ll i = 1;i<=sz[e];i++){
ll cur = 0;
for(ll j = sz[f];j>=1;j--){
calc[e][f][i][j] = cur;
if(pos[e][i]>pos[f][j]) cur++;
}
calc[e][f][i][0] = cur;
}
}
}
ll ans = llinf;
ans = min(ans,reshi(sz[1],sz[2],sz[3],1));
ans = min(ans,reshi(sz[1],sz[2],sz[3],2));
ans = min(ans,reshi(sz[1],sz[2],sz[3],3));
if(ans==llinf) ans = -1;
cout<<ans<<endl;
return 0;
}
