#include<bits/stdc++.h>
using namespace std;
int n, rr, yy, gg;
vector<int> r, y, g;
vector<vector<vector<int>>> lR, lY, lG;
void cal(int pR, int pY, int pG, char prev) {
int pos=pR+pY+pG;
if(pos == n) {
if(prev == 'R') lR[pR][pY][pG]=0;
else if(prev == 'Y') lY[pR][pY][pG]=0;
else lG[pR][pY][pG]=0;
return;
}
//cout << pos << ' ' << pR << ' ' << pY << ' ' << pG << ' ' << prev << endl;
int newPR=(pR == rr ? -1 : r[pR]), newPY=(pY == yy ? -1 : y[pY]), newPG=(pG == gg ? -1 : g[pG]);
for(int i=0; i<pR; i++) {
if(pY != yy && r[i] > y[pY]) newPY++;
if(pG != gg && r[i] > g[pG]) newPG++;
}
for(int i=0; i<pY; i++) {
if(pR != rr && y[i] > r[pR]) newPR++;
if(pG != gg && y[i] > g[pG]) newPG++;
}
for(int i=0; i<pG; i++) {
if(pY != yy && g[i] > y[pY]) newPY++;
if(pR != rr && g[i] > r[pR]) newPR++;
}
//cout << newPR << ' ' << newPY << ' ' << newPG << endl;
//return;
if(prev == 'R' || prev == ' ') {
if(pY != yy) {
cal(pR, pY+1, pG, 'Y');
lR[pR][pY][pG]=min(lR[pR][pY][pG], lY[pR][pY+1][pG]+max(newPY-pos, 0));
}
if(pG != gg) {
cal(pR, pY, pG+1, 'G');
lR[pR][pY][pG]=min(lG[pR][pY][pG], lG[pR][pY][pG+1]+max(newPG-pos, 0));
}
}
if(prev == 'Y' || prev == ' ') {
if(pR != rr) {
cal(pR+1, pY, pG, 'R');
lY[pR][pY][pG]=min(lY[pR][pY][pG], lR[pR+1][pY][pG]+max(newPR-pos, 0));
}
if(pG != gg) {
cal(pR, pY, pG+1, 'G');
lY[pR][pY][pG]=min(lY[pR][pY][pG], lG[pR][pY][pG+1]+max(newPG-pos, 0));
}
}
if(prev == 'G' || prev == ' ') {
if(pY != yy) {
cal(pR, pY+1, pG, 'Y');
lG[pR][pY][pG]=min(lG[pR][pY][pG], lY[pR][pY+1][pG]+max(newPY-pos, 0));
}
if(pR != rr) {
cal(pR+1, pY, pG, 'R');
lG[pR][pY][pG]=min(lG[pR][pY][pG], lR[pR+1][pY][pG]+max(newPR-pos, 0));
}
}
}
signed main() {
cin >> n;
string s; cin >> s;
for(int i=0; i<n; i++) {
if(s[i] == 'R') r.push_back(i);
else if(s[i] == 'Y') y.push_back(i);
else g.push_back(i);
}
rr=r.size(); yy=y.size(); gg=g.size();
if(rr > yy+gg+1 || yy > rr+gg+1 || gg > rr+yy+1) {
cout << "-1"; return 0;
}
lR=lY=lG=vector<vector<vector<int>>>(rr+1, vector<vector<int>>(yy+1, vector<int>(gg+1, 1e5)));
cal(0, 0, 0, ' ');
int ans = min(lR[0][0][0], min(lG[0][0][0], lY[0][0][0]));
if(ans == 1e5) ans=-1;
cout << ans;
return 0;
}
