#include <bits/stdc++.h>
 
//    #include <boost/multiprecision/cpp_int.hpp>
 //#define int long long
 #define inf  1000000007
 #define pa pair<int,int>
 #define ll long long
 #define pal pair<double,double>
 #define ppap pair<pa,int>
  #define PI 3.14159265358979323846
  #define paa pair<int,char>
  #define  mp make_pair
  #define  pb push_back
  #define EPS (1e-8)
                                          
    int dx[8]={0,1,0,-1,1,1,-1,-1};
    int dy[8]={1,0,-1,0,-1,1,1,-1};
                                            using namespace std;
                                   			class pa3{
                                            	public:
                                            	int x;
                                   				int y,z;
                                            	pa3(int x=0,int y=0,int z=0):x(x),y(y),z(z) {}
                                            	bool operator < (const pa3 &p) const{
                                            		if(x!=p.x) return x<p.x;
                                            		if(y!=p.y) return y<p.y;
                                            		 return z<p.z;
                                            		//return x != p.x ? x<p.x: y<p.y;
                                            	}
                                   				bool operator > (const pa3 &p) const{
                                            		if(x!=p.x) return x>p.x;
                                            		if(y!=p.y) return y>p.y;
                                            		 return z>p.z;
                                            		//return x != p.x ? x<p.x: y<p.y;
                                            	}
                                            	bool operator == (const pa3 &p) const{
                                            		return x==p.x && y==p.y && z==p.z;
                                            	}
                                            		bool operator != (const pa3 &p) const{
                                            			return !( x==p.x && y==p.y && z==p.z);
                                            	}
                                            
                                            };
                                            
                                            class pa4{
                                            	public:
                                            	int x;
                                            	int y,z,w;
                                            	pa4(int x=0,int y=0,int z=0,int w=0):x(x),y(y),z(z),w(w) {}
                                            	bool operator < (const pa4 &p) const{
                                            		if(x!=p.x) return x<p.x;
                                            		if(y!=p.y) return y<p.y;
                                            		if(z!=p.z)return z<p.z;
                                            		return w<p.w;
                                            		//return x != p.x ? x<p.x: y<p.y;
                                            	}
                                            	bool operator > (const pa4 &p) const{
                                            		if(x!=p.x) return x>p.x;
                                            		if(y!=p.y) return y>p.y;
                                            		if(z!=p.z)return z>p.z;
                                            		return w>p.w;
                                            		//return x != p.x ? x<p.x: y<p.y;
                                            	}
                                            	bool operator == (const pa4 &p) const{
                                            		return x==p.x && y==p.y && z==p.z &&w==p.w;
                                            	}
                                            		
                                            
                                            };
                                            class pa2{
                                            	public:
                                            	int x,y;
                                            	pa2(int x=0,int y=0):x(x),y(y) {}
                                            	pa2 operator + (pa2 p) {return pa2(x+p.x,y+p.y);}
                                            	pa2 operator - (pa2 p) {return pa2(x-p.x,y-p.y);}
                                            	bool operator < (const pa2 &p) const{
                                            		return y != p.y ? y<p.y: x<p.x;
                                            	}
                                            	bool operator > (const pa2 &p) const{
                                            		return x != p.x ? x<p.x: y<p.y;
                                            	}
                                            	bool operator == (const pa2 &p) const{
                                            		return abs(x-p.x)==0 && abs(y-p.y)==0;
                                            	}
                                            	bool operator != (const pa2 &p) const{
                                            		return !(abs(x-p.x)==0 && abs(y-p.y)==0);
                                            	}
                                            		
                                            
                                            };
                                            
 
                      
                                string itos( int i ) {
                                ostringstream s ;
                                s << i ;
                                return s.str() ;
                                }
                                 
                                int gcd(int v,int b){
                                	if(v==0) return b;
                                	if(b==0) return v;
                                	if(v>b) return gcd(b,v);
                                	if(v==b) return b;
                                	if(b%v==0) return v;
                                	return gcd(v,b%v);
                                }
                 
                            
                                int mod;
int extgcd(int a, int b, int &x, int &y) {
    if (b == 0) {
        x = 1;
        y = 0;
        return a;
    }
    int d = extgcd(b, a%b, y, x);
    y -= a/b * x;
    return d;
}
pa operator+(const pa & l,const pa & r) {   
    return {l.first+r.first,l.second+r.second};                                    
}    
pa operator-(const pa & l,const pa & r) {   
    return {l.first-r.first,l.second-r.second};                                    
}  
                                
                int pr[10000100];
                int inv[10000010];
                
                int beki(int wa,int rr,int warukazu){
                	if(rr==0) return 1%warukazu;
                	if(rr==1) return wa%warukazu;
                	wa%=warukazu;
                	if(rr%2==1) return ((ll)beki(wa,rr-1,warukazu)*(ll)wa)%warukazu;
                	ll zx=beki(wa,rr/2,warukazu);
                	return (zx*zx)%warukazu;
                }
 
                
    			int comb(int nn,int rr){
    				if(rr<0 || rr>nn || nn<0) return 0;
    				int r=pr[nn]*inv[rr];
    				r%=mod;
    				r*=inv[nn-rr];
    				r%=mod;
    				return r;
    			}
               
                void gya(int ert){
                	pr[0]=1;
                	for(int i=1;i<=ert;i++){
                		pr[i]=(pr[i-1]*i)%mod;
                	}
                		inv[ert]=beki(pr[ert],mod-2,mod);
                	for(int i=ert-1;i>=0;i--){
                		inv[i]=inv[i+1]*(i+1)%mod;
                	}
                }
                
              //   cin.tie(0);
    		//	ios::sync_with_stdio(false);
    			//priority_queue<pa3,vector<pa3>,greater<pa3>> pq;            
                 //sort(ve.begin(),ve.end(),greater<int>());
 //   mt19937(clock_per_sec);
  //  mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count()) ;
      

int dp[410][410][410][3];
string s;

int memo[410][3]={};
vector<int> ve[3];

int solve(int a,int b,int c,int t){
	if(a<0 || b<0 || c<0) return inf;
	if(a==0 && b==0 && c==0) return 0;
	if(dp[a][b][c][t]>=0) return dp[a][b][c][t];
	if(t==0 && a==0)dp[a][b][c][t]=inf;
	if(t==1 && b==0)dp[a][b][c][t]=inf;
	if(t==2 && c==0)dp[a][b][c][t]=inf;
	if(dp[a][b][c][t]>=0) return dp[a][b][c][t];
	int p=inf,q=inf;
	if(t==0)if(a>=1){
		int T=max(0,memo[ve[0][a-1]][2]-c)+max(0,memo[ve[0][a-1]][1]-b);
		p=solve(a-1,b,c,1)+T;
		q=solve(a-1,b,c,2)+T;
		
	}
	if(t==1)if(b>=1){
		int T=max(0,memo[ve[1][b-1]][0]-a)+max(0,memo[ve[1][b-1]][2]-c);
		if(b>=1)p=solve(a,b-1,c,0)+T;
		if(b>=1)q=solve(a,b-1,c,2)+T;
	}
	if(t==2)if(c>=1){
		int T=max(0,memo[ve[2][c-1]][0]-a)+max(0,memo[ve[2][c-1]][1]-b);
		if(c>=1)p=solve(a,b,c-1,0)+T;
		if(c>=1)q=solve(a,b,c-1,1)+T;
	}
	//cout<<a<<" "<<b<<" "<<c<<" "<<t<<"   "<<p<<" "<<q<<endl;
	p=min(p,inf);
dp[a][b][c][t]=min(p,q);
return min(p,q);
	
	
	
}

signed main(){
	
	
   cin.tie(0);
	ios::sync_with_stdio(false);
	int n;
	cin>>n;
	cin>>s;
	
	int t1=0,t2=0,t3=0;
	int yy=0;
	for(auto c:s){
		t1+=c=='R';
		t2+=c=='G';
		t3+=c=='Y';
		if(c=='R')memo[yy][0]++,ve[0].pb(yy);
		if(c=='G')memo[yy][1]++,ve[1].pb(yy);
		if(c=='Y')memo[yy][2]++,ve[2].pb(yy);
		yy++;
	}
	for(int i=1;i<n;i++)for(int j=0;j<3;j++)memo[i][j]+=memo[i-1][j];
	
	for(int i=0;i<=t1;i++)for(int j=0;j<=t2;j++)for(int k=0;k<=t3;k++)for(int h=0;h<3;h++)dp[i][j][k][h]=-1;
	
		int y=min({solve(t1,t2,t3,0),solve(t1,t2,t3,1),solve(t1,t2,t3,2)});
	if(y>=inf)cout<<-1<<endl;
	else cout<<y<<endl;
	
	return 0;
	

 
 }



Measured memory_bytes: 165164