#include<atcoder/all>
using namespace std;
using namespace atcoder;

int op(int l,int r){return min(l,r);}
int e(){return 4;}

int main(){
    int n;
    string s;
    cin >> n >> s;
    int ans = n;
    array<vector<int>,3> tb;
    array<int,3> b={0,1,2};
    auto f=[&](char c){
        if(c == 'R')return 0;
        if(c == 'G')return 1;
        if(c == 'B')return 2;
    };
    for(int i = 0; i < n; i++) tb[f(s[i])].push_back(i);
    do{
        segtree<int,op,e> seg(n);
        dsu uf(n);
        for(int i = 0; i < 3; i++){
            for(auto j:tb[i])seg.set(j, b[i]);
        }
        for(int i = 0; i < 3; i++){
            for(int j = 0; j + 1 < tb[i].size(); j++){
                if(seg.prod(tb[i][j], tb[i][j+1]) >= b[i])uf.merge(tb[i][j], tb[i][j+1]);
            }
        }
        ans = min(ans, int(uf.groups().size()));
    }while(next_permutation(b.begin(), b.end()));
    cout << ans << endl;
}