#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#define AquA cin.tie(0);ios_base::sync_with_stdio(0);
#define fs first
#define sc second
#define cd complex<double>
#define p_q priority_queue
#define vi vector<int>
#define vvi vector<vi>
#define vvvi vector<vvi>
#define vvvvi vector<vvvi>
using namespace std;
const int inf=1e9+7;
int main(){
AquA;
int n;
cin >> n;
string s;
cin >> s;
int cr=0,cb=0,cg=0;
vi rr,bb,gg,tg(n+1),tb(n+1),tr(n+1);
rr.push_back(-1);
bb.push_back(-1);
gg.push_back(-1);
for(int i=0;i<n;i++){
tr[i]=tr[(i-1)*(i>0)];
tb[i]=tb[(i-1)*(i>0)];
tg[i]=tg[(i-1)*(i>0)];
if(s[i]=='R'){
tr[i]++;
cr++;
rr.push_back(i);
}
else if(s[i]=='Y'){
tb[i]++;
cb++;
bb.push_back(i);
}
else{
tg[i]++;
cg++;
gg.push_back(i);
}
}
vvvvi dp(cr+1,vvvi(cb+1,vvi(cg+1,vi(3,inf))));
dp[0][0][0][0]=dp[0][0][0][1]=dp[0][0][0][2]=0;
for(int r=0;r<=cr;r++){
for(int b=0;b<=cb;b++){
for(int g=0;g<=cg;g++){
if(r>0){
int c=max(0,tg[rr[r]]-g)+max(0,tb[rr[r]]-b);
dp[r][b][g][0]=min(dp[r][b][g][0],min(dp[r-1][b][g][1],dp[r-1][b][g][2])+c);
}
if(b>0){
int c=max(0,tg[bb[b]]-g)+max(0,tr[bb[b]]-r);
dp[r][b][g][1]=min(dp[r][b][g][1],min(dp[r][b-1][g][0],dp[r][b-1][g][2])+c);
}
if(g>0){
int c=max(0,tb[gg[g]]-b)+max(0,tr[gg[g]]-r);
dp[r][b][g][2]=min(dp[r][b][g][2],min(dp[r][b][g-1][0],dp[r][b][g-1][1])+c);
}
//cout << r << " " << b << " " << g << " " << dp[r][b][g][0] << " " << dp[r][b][g][1] << " " << dp[r][b][g][2] << "\n";
}
}
}
int h=min(dp[cr][cb][cg][0],min(dp[cr][cb][cg][1],dp[cr][cb][cg][2]));
if(h==inf){
cout << -1 << "\n";
}
else{
cout << h << "\n";
}
return 0;
}
