#include "bits/stdc++.h"
using namespace std;
const int maxn = 405;
int n, a[maxn];
string s;
int pref[maxn][3];
long long f[maxn][maxn][maxn][3];
vector<int> pos[maxn];
int convert(char c) {
if(c == 'R') return 0;
if(c == 'G') return 1;
if(c == 'Y') return 2;
return -1;
}
void solve() {
cin >> n >> s;
s = ' ' + s;
for(int i = 1; i <= n; ++i) {
a[i] = convert(s[i]);
for(int j = 0; j < 3; ++j) {
pref[i][j] = pref[i - 1][j];
}
pref[i][a[i]]++;
pos[a[i]].push_back(i);
}
for(int i = 0; i <= pref[n][0]; ++i) {
for(int j = 0; j <= pref[n][1]; ++j) {
for(int k = 0; k <= pref[n][2]; ++k) {
for(int x = 0; x < 3; ++x) f[i][j][k][x] = 1e9;
}
}
}
for(int i = 0; i < 3; ++i) f[0][0][0][i] = 0;
for(int i = 0; i < n; ++i) {
for(int j = 0; j <= min(pref[n][0], i); ++j) {
for(int k = 0; k <= min(pref[n][1], i - j); ++k) {
int l = i - j - k;
if(l > pref[n][2]) continue;
if(j < pref[n][0]) {
long long cur = min(f[j][k][l][1], f[j][k][l][2]);
cur += max(0, pref[pos[0][j]][1] - k) + max(0, pref[pos[0][j]][2] - l);
f[j + 1][k][l][0] = min(f[j + 1][k][l][0], cur);
}
if(k < pref[n][1]) {
long long cur = min(f[j][k][l][0], f[j][k][l][2]);
cur += max(0, pref[pos[1][k]][0] - j) + max(0, pref[pos[1][k]][2] - l);
f[j][k + 1][l][1] = min(f[j][k + 1][l][1], cur);
}
if(l < pref[n][2]) {
long long cur = min(f[j][k][l][1], f[j][k][l][0]);
cur += max(0, pref[pos[2][l]][1] - k) + max(0, pref[pos[2][l]][0] - j);
f[j][k][l + 1][2] = min(f[j][k][l + 1][2], cur);
}
}
}
}
long long res = 1e9;
for(int i = 0; i < 3; ++i) {
res = min(res, f[pref[n][0]][pref[n][1]][pref[n][2]][i]);
}
cout << (res == 1e9 ? -1 : res);
}
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
solve();
return 0;
}
