#include <cstdio>
#include <vector>
#include <cmath>
#include <algorithm>
#include <cstring>
using namespace std;
int n;
char arr[405];
int pre[405][3];
vector<int> r,g,y;
int memo[205][205][205][3];
const int INF=1000000000;
int dp(int sr,int sg,int sy,int last){ ///1000000000 for invalid case
if (!sr && !sg && !sy) return 0;
else if (memo[sr][sg][sy][last]!=-1) return memo[sr][sg][sy][last];
int ans=INF;
if (last!=0 && sr) ans=min(ans,dp(sr-1,sg,sy,0)
+max(0,pre[r[sr-1]][1]-sg)+max(0,pre[r[sr-1]][2]-sy));
if (last!=1 && sg) ans=min(ans,dp(sr,sg-1,sy,1)
+max(0,pre[g[sg-1]][0]-sr)+max(0,pre[g[sg-1]][2]-sy));
if (last!=2 && sy) ans=min(ans,dp(sr,sg,sy-1,2)
+max(0,pre[y[sy-1]][0]-sr)+max(0,pre[y[sy-1]][1]-sg));
//printf("%d %d %d %d %d\n",sr,sg,sy,last,ans);
return memo[sr][sg][sy][last]=ans;
}
int main(){
//freopen("input.txt","r",stdin);
scanf("%d",&n);
getchar();
for (int x=0;x<n;x++){
arr[x]=getchar();
if (arr[x]=='R') r.push_back(x);
else if (arr[x]=='G') g.push_back(x);
else y.push_back(x);
}
if (r.size()>200 || g.size()>200 || y.size()>200) {
printf("-1\n");
return 0;
}
for (int x=0;x<n;x++){
if (arr[x]=='R') pre[x][0]++;
else if (arr[x]=='G') pre[x][1]++;
else pre[x][2]++;
for (int y=0;y<3;y++) pre[x+1][y]=pre[x][y];
}
memset(memo,-1,sizeof(memo));
int ans=INF;
if (!r.empty()) ans=min(ans,dp(r.size()-1,g.size(),y.size(),0));
if (!g.empty()) ans=min(ans,dp(r.size(),g.size()-1,y.size(),1));
if (!y.empty()) ans=min(ans,dp(r.size(),g.size(),y.size()-1,2));
printf("%d\n",(ans==INF)?-1:ans);
}
