//#pragma GCC optimize("O3")
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long double
#define pb push_back
#define ff first
#define ss second
#define MOD 1000000009
#define INF 1000000019
#define INFL 1000000000000000099LL
ll n,q,s,t,a,b,c,d,ans,bst,k,e,m,pier,h,w,u;
char ch;
ll dp[407][407][407][3];
vector<ll>v,pr[3];
ll gdzie[407][3];//gdzie ity jtego koloru
int main()
{
ios_base::sync_with_stdio(0);cin.tie(0);
cin>>n;
pr[0].pb(0);
pr[1].pb(0);
pr[2].pb(0);
for(ll i=0;i<n;i++){
cin>>ch;
if(ch=='R')
v.pb(0);
else if(ch=='Y')
v.pb(1);
else
v.pb(2);
for(ll j =0;j<3;j++)pr[j].pb(pr[j].back());
pr[v.back()].back()++;
gdzie[pr[v.back()].back()][v.back()]=i+1;
}//return 0;
for(ll i=0;i<=n+1;i++){
for(ll j=0;j<=n+1;j++){
for(ll k=0;k<3;k++){
dp[i][j][0][k]=INFL;
dp[i][0][j][k]=INFL;
dp[0][i][j][k]=INFL;
}
}
}//return 0;
for(ll i=1;i<=n+1;i++){
for(ll j=1;j<=n+1;j++){
for(ll k=1;k<=n+1;k++){
if(1==i && 1==j && 1==k){
continue;
}
for(ll l=0;l<3;l++){
dp[i][j][k][l]=INFL;
ll pom;
if(l==0)pom=i-1;
else if(l==1)pom=j-1;
else pom=k-1;
if(l!=0){
dp[i][j][k][l]=min(dp[i][j][k][l],dp[i-(l==0)][j-(l==1)][k-(l==2)][0]+max(0LL,(ll)pr[1][gdzie[pom][l]]-j+1)+max(0LL,(ll)pr[2][gdzie[pom][l]]-k+1)+max(0LL,(ll)pr[0][gdzie[pom][l]]-i+1));
}
if(l!=1){
dp[i][j][k][l]=min(dp[i][j][k][l],dp[i-(l==0)][j-(l==1)][k-(l==2)][1]+max(0LL,(ll)pr[1][gdzie[pom][l]]-j+1)+max(0LL,(ll)pr[2][gdzie[pom][l]]-k+1)+max(0LL,(ll)pr[0][gdzie[pom][l]]-i+1));
}
if(l!=2){
dp[i][j][k][l]=min(dp[i][j][k][l],dp[i-(l==0)][j-(l==1)][k-(l==2)][2]+max(0LL,(ll)pr[1][gdzie[pom][l]]-j+1)+max(0LL,(ll)pr[2][gdzie[pom][l]]-k+1)+max(0LL,(ll)pr[0][gdzie[pom][l]]-i+1));
}
//cout<<i<<" "<<j<<" "<<k<<" "<<l<<" "<<dp[i][j][k][l]<<"\n";
}
}
}
}
pr[0].back()++;
pr[1].back()++;
pr[2].back()++;
if(min(dp[pr[0].back()][pr[1].back()][pr[2].back()][0],min(dp[pr[0].back()][pr[1].back()][pr[2].back()][2],dp[pr[0].back()][pr[1].back()][pr[2].back()][1]))>1000000000)cout<<-1;
else
cout<<min(dp[pr[0].back()][pr[1].back()][pr[2].back()][0],min(dp[pr[0].back()][pr[1].back()][pr[2].back()][2],dp[pr[0].back()][pr[1].back()][pr[2].back()][1]));
}
