#include "bits/stdc++.h"
#pragma optimize ("Bismillahirrahmanirrahim")
using namespace std;
#define pb push_back
#define ff first
#define ss second
#define endl "\n"
//#define int long long
#define double long double
#define sz(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define what_is(x) cerr << #x << " is " << x << endl;
//#define m (l+r)/2
constexpr int N=200005;
constexpr int MOD=1000000007;
//constexpr int INF2 = LLONG_MAX;
constexpr int INF=(int)1e9;
constexpr int LOG=30;
typedef pair<int,int> pii;
typedef tuple<int,int,int> tp;
typedef priority_queue<pii,vector<pii>,greater<pii>> min_pq;
typedef priority_queue<pii> max_pq;
typedef long long ll;
//to think//
/*
* graph approach
* dp
* dividing the problem to smaller statements
* finding the real constraint
* sqrt decomposition
* greedy approach
* pigeonhole principle
* rewriting the problem/equality
* bitwise approaches
* binary search if monotonic
* divide and conquer
* combinatorics
* inclusion - exclusion
* think like bfs
*/
inline int in()
{
int x;cin >> x;
return x;
}
inline string in2()
{
string x;cin >> x;
return x;
}
string s;
int n;
vector<int> poz[3];
int dp[401][401][401][4];
int pre[404][3];
inline int cost(int a,int b,int c,int r)
{
int res=0;
if(r==0)
{
res+=max(0,pre[poz[0][a]][1]-(b==0 ? 0 : pre[poz[1][b-1]][1]));
res+=max(0,pre[poz[0][a]][2]-(c==0 ? 0 : pre[poz[2][c-1]][2]));
}
else if(r==1)
{
res+=max(0,pre[poz[1][b]][0]-(a==0 ? 0 : pre[poz[0][a-1]][0]));
res+=max(0,pre[poz[1][b]][2]-(c==0 ? 0 : pre[poz[2][c-1]][2]));
}
else
{
res+=max(0,pre[poz[2][c]][0]-(a==0 ? 0 : pre[poz[0][a-1]][0]));
res+=max(0,pre[poz[2][c]][1]-(b==0 ? 0 : pre[poz[1][b-1]][1]));
}
return max(0,res);
}
inline int f(int a,int b,int c,int fl)
{
if(a+b+c>n) return INF;
if(a+b+c==n) return 0;
if(dp[a][b][c][fl]!=-1) return dp[a][b][c][fl];
if(fl==0)
{
if(b==sz(poz[1]) && c==sz(poz[2])) return dp[a][b][c][fl]=INF;
int cev1=INF;
if(b<sz(poz[1])) cev1=min(cev1,f(a,b+1,c,1)+cost(a,b,c,1));
if(c<sz(poz[2])) cev1=min(cev1,f(a,b,c+1,2)+cost(a,b,c,2));
return dp[a][b][c][fl]=cev1;
}
else if(fl==1)
{
if(a==sz(poz[0]) && c==sz(poz[2])) return dp[a][b][c][fl]=INF;
int cev1=INF;
if(a<sz(poz[0])) cev1=min(cev1,f(a+1,b,c,0)+cost(a,b,c,0));
if(c<sz(poz[2])) cev1=min(cev1,f(a,b,c+1,2)+cost(a,b,c,2));
return dp[a][b][c][fl]=cev1;
}
else if(fl==2)
{
if(a==sz(poz[0]) && b==sz(poz[1])) return dp[a][b][c][fl]=INF;
int cev1=INF;
if(a<sz(poz[0])) cev1=min(cev1,f(a+1,b,c,0)+cost(a,b,c,0));
if(b<sz(poz[1])) cev1=min(cev1,f(a,b+1,c,1)+cost(a,b,c,1));
return dp[a][b][c][fl]=cev1;
}
if(a==sz(poz[0]) && b==sz(poz[1]) && c==sz(poz[2])) return dp[a][b][c][fl]=INF;
int cev1=INF;
if(a<sz(poz[0])) cev1=min(cev1,f(a+1,b,c,0)+cost(a,b,c,0));
if(b<sz(poz[1])) cev1=min(cev1,f(a,b+1,c,1)+cost(a,b,c,1));
if(c<sz(poz[2])) cev1=min(cev1,f(a,b,c+1,2)+cost(a,b,c,2));
return dp[a][b][c][fl]=cev1;
}
void solve()
{
memset(dp,-1,sizeof(dp));
n=in();s=in2();
s="$"+ s;
for(int i=1;i<=n;i++)
{
pre[i][0]=pre[i-1][0];
pre[i][1]=pre[i-1][1];
pre[i][2]=pre[i-1][2];
if(s[i]=='R') {poz[0].pb(i);pre[i][0]++;}
else if(s[i]=='G') {poz[1].pb(i);pre[i][1]++;}
else {poz[2].pb(i);pre[i][2]++;}
}
int ans=f(0,0,0,3);
if(ans==INF) cout << -1 << endl;
else cout << ans << endl;
}
int32_t main(){
cin.tie(0); ios::sync_with_stdio(0);
cout << fixed << setprecision(15);
int t=1;//cin>> t;
for(int i=1;i<=t;i++)
{
// cout << "Case #" << i << ": ";
solve();
}
return 0;
}
