#include <bits/stdc++.h>
#define ll long long
#define ar array
#define db double
#define all(x) x.begin(), x.end()
#define sz(x) (int)x.size()
using namespace std;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
#define rint(l, r) uniform_int_distribution<int>(l, r)(rng)
template<typename T> bool ckmin(T &a, const T &b) { return a > b ? a = b, 1 : 0; }
template<typename T> bool ckmax(T &a, const T &b) { return a < b ? a = b, 1 : 0; }
const int INF = 1e9;
void test_case() {
int n; string s;
cin >> n >> s;
vector<int> a(n);
for (int i = 0; i < n; i++) {
a[i] = (s[i] == 'R' ? 0 : s[i] == 'G' ? 1 : 2);
}
ar<int, 3> on{};
int where[3][n+1];
memset(where, -1, sizeof(where));
for (int i = 0; i < n; i++) {
where[a[i]][on[a[i]]++] = i;
}
int dp[n+1][n+1][n+1][3];
fill(&dp[0][0][0][0], &dp[0][0][0][0] + 3*(n+1)*(n+1)*(n+1), INF);
memset(dp[0][0][0], 0, sizeof(dp[0][0][0]));
for (int s = 0; s < n; s++) {
for (int i = 0; i <= min(on[0], s); i++) {
for (int j = 0; j <= on[1] && i+j <= s; j++) {
ar<int, 3> cur{i, j, s-i-j};
for (int k = 0; k < 3; k++) {
if (dp[i][j][s-i-j][k] == INF) continue;
for (int nw = 0; nw < 3; nw++) if (nw != k) {
int pos = where[nw][cur[nw]];
if (~pos) {
int cnt = 0;
for (int z = 0; z < 3; z++) {
for (int idx = 0; idx < cur[z]; idx++) {
cnt += where[z][idx] > pos;
}
}
ckmin(dp[i+(nw==0)][j+(nw==1)][s-i-j+(nw==2)][nw], dp[i][j][s-i-j][k] + cnt);
}
}
}
}
}
}
int ans = INF;
for (int k = 0; k < 3; k++) {
ckmin(ans, dp[on[0]][on[1]][on[2]][k]);
}
cout << (ans == INF ? -1 : ans) << '\n';
}
int main() {
cin.tie(0)->sync_with_stdio(0);
int t = 1;
// cin >> t;
while (t--) test_case();
}
