#include <iostream>
#include <bits/stdc++.h>
#define ll long long
using namespace std;
const ll maxn = 2*1e5+5, INF = 1e9+9;
int f[401][401][401][3];
void solve(){
int n;
string s;
cin >> n;
cin >> s;
s = ' ' + s;
int R = 0, G = 0, Y = 0;
vector<int> pfr(n+1, 0), pfg(n+1, 0), pfy(n+1, 0);
vector<int> pr(n+1, 0), pg(n+1, 0), py(n+1, 0);
for(int i = 1; i <= n; i++){
pfr[i] = pfr[i-1];
pfg[i] = pfg[i-1];
pfy[i] = pfy[i-1];
if(s[i] == 'R'){
R++;
pr[R] = i;
pfr[i]++;
}
if(s[i] == 'G'){
G++;
pg[G] = i;
pfg[i]++;
}
if(s[i] == 'Y'){
Y++;
py[Y] = i;
pfy[i]++;
}
}
auto chmin = [&](int &a, int b) -> void{
a = min(a, b);
};
for(int r = 0; r <= R; r++){
for(int g = 0; g <= G; g++){
for(int y = 0; y <= Y; y++){
for(int k = 0; k < 3; k++){
f[r][g][y][k] = INF;
}
}
}
}
for(int k = 0; k < 3; k++){
f[0][0][0][k] = 0;
}
for(int i = 1; i <= n; i++){
for(int r = 0; r <= R; r++){
for(int g = 0; g <= G; g++){
int y = i-r-g;
if(y < 0 || y > Y) continue;
if(r > 0){
chmin(f[r][g][y][0], min(f[r-1][g][y][1], f[r-1][g][y][2]) + pr[r]+max(0, g-pfg[pr[r]]) + max(0, y-pfy[pr[r]])-i);
}
if(g > 0){
chmin(f[r][g][y][1], min(f[r][g-1][y][0], f[r][g-1][y][2]) + pg[g]+max(0, r-pfr[pg[g]]) + max(0, y-pfy[pg[g]])-i);
}
if(y > 0){
chmin(f[r][g][y][2], min(f[r][g][y-1][0], f[r][g][y-1][1]) + py[y]+max(0, r-pfr[py[y]]) + max(0, g-pfg[py[y]])-i);
}
}
}
}
int ans = INF;
for(int k = 0; k < 3; k++){
chmin(ans, f[R][G][Y][k]);
}
if(ans > 1e8) ans = -1;
cout << ans;
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
solve();
}
