#ifndef CPL_TEMPLATE
#define CPL_TEMPLATE
/*
Normie's Template v2.5
Changes:
Added warning against using pragmas on USACO.
*/
// Standard library in one include.
#include <bits/stdc++.h>
using namespace std;
// ordered_set library.
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
#define ordered_set(el) tree<el,null_type,less<el>,rb_tree_tag,tree_order_statistics_node_update>
// AtCoder library. (Comment out these two lines if you're not submitting in AtCoder.) (Or if you want to use it in other judges, run expander.py first.)
//#include <atcoder/all>
//using namespace atcoder;
//Pragmas (Comment out these three lines if you're submitting in szkopul or USACO.)
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast,unroll-loops,tree-vectorize")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
//File I/O.
#define FILE_IN "cseq.inp"
#define FILE_OUT "cseq.out"
#define ofile freopen(FILE_IN,"r",stdin);freopen(FILE_OUT,"w",stdout)
//Fast I/O.
#define fio ios::sync_with_stdio(0);cin.tie(0)
#define nfio cin.tie(0)
#define endl "\n"
//Order checking.
#define ord(a,b,c) ((a>=b)and(b>=c))
//min/max redefines, so i dont have to resolve annoying compile errors.
#define min(a,b) (((a)<(b))?(a):(b))
#define max(a,b) (((a)>(b))?(a):(b))
// Fast min/max assigns to use with AVX.
// Requires g++ 9.2.0.
template<typename T>
__attribute__((always_inline)) void chkmin(T& a, const T& b) {
a=(a<b)?a:b;
}
template<typename T>
__attribute__((always_inline)) void chkmax(T& a, const T& b) {
a=(a>b)?a:b;
}
//Constants.
#define MOD (ll(998244353))
#define MAX 300001
#define mag 320
const long double PI=3.14159265358979;
//Pairs and 3-pairs.
#define p1 first
#define p2 second.first
#define p3 second.second
#define fi first
#define se second
#define pii(element_type) pair<element_type,element_type>
#define piii(element_type) pair<element_type,pii(element_type)>
//Quick power of 2.
#define pow2(x) (ll(1)<<x)
//Short for-loops.
#define ff(i,__,___) for(int i=__;i<=___;i++)
#define rr(i,__,___) for(int i=__;i>=___;i--)
//Typedefs.
#define bi BigInt
typedef long long ll;
typedef long double ld;
typedef short sh;
// Binpow and stuff
ll BOW(ll a, ll x, ll p)
{
if (!x) return 1;
ll res=BOW(a,x/2,p);
res*=res;
res%=p;
if (x%2) res*=a;
return res%p;
}
ll INV(ll a, ll p)
{
return BOW(a,p-2,p);
}
//---------END-------//
#endif
vector<int> vec;
int dp[401][401][401][3];
int n,m,i,j,k,t,t1,u,v,a,b;
int bef[401][3][3];
int cnt[3];
int arr[401];
int main()
{
fio;
cin>>n;
char c;
for (i=0;i<n;i++)
{
cin>>c;
if (c=='R') arr[i]=0;
if (c=='G') arr[i]=1;
if (c=='Y') arr[i]=2;
cnt[arr[i]]++;
}
for (i=0;i<3;i++) for (j=0;j<3;j++)
{
u=0;
v=0;
for (k=0;k<n;k++)
{
if (arr[k]==i)
{
bef[u][i][j]=v;
u++;
}
if (arr[k]==j)
{
bef[v][j][i]=u;
v++;
}
}
}
for (i=0;i<=cnt[0];i++) for (j=0;j<=cnt[1];j++) for (k=0;k<=cnt[2];k++)
{
dp[i][j][k][0]=1e9;
dp[i][j][k][1]=1e9;
dp[i][j][k][2]=1e9;
}
dp[0][0][1][2]=0;
dp[0][1][0][1]=0;
dp[1][0][0][0]=0;
for (i=0;i<=cnt[0];i++) for (j=0;j<=cnt[1];j++) for (k=0;k<=cnt[2];k++)
{
if (i+j+k==n) break;
if (i+j+k)
{
for (t=0;t<3;t++)
{
// cout<<i<<' '<<j<<' '<<k<<' '<<t<<' '<<dp[i][j][k][t]<<endl;
if ((i-cnt[0])and(t-0)) dp[i+1][j][k][0]=min(dp[i+1][j][k][0],dp[i][j][k][t]+max(0,j-bef[i][0][1])+max(0,k-bef[i][0][2]));
if ((j-cnt[1])and(t-1)) dp[i][j+1][k][1]=min(dp[i][j+1][k][1],dp[i][j][k][t]+max(0,i-bef[j][1][0])+max(0,k-bef[j][1][2]));
if ((k-cnt[2])and(t-2)) dp[i][j][k+1][2]=min(dp[i][j][k+1][2],dp[i][j][k][t]+max(0,i-bef[k][2][0])+max(0,j-bef[k][2][1]));
}
}
}
a=1e9;
for (t=0;t<3;t++)
{
a=min(a,dp[cnt[0]][cnt[1]][cnt[2]][t]);
}
if (a==1e9) cout<<-1; else cout<<a;
}
// Normie28
