//Sylwia Sapkowska
#include <bits/stdc++.h>
#pragma GCC optimize("O3", "unroll-loops")
using namespace std;
void __print(int x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << "'" << x << "'";}
void __print(const char *x) {cerr << '"' << x << '"';}
void __print(const string &x) {cerr << '"' << x << '"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ", "; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? ", " : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifdef LOCAL
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif
// #define int long long
typedef pair<int, int> T;
const int oo = 1e9+7, oo2 = 1e9+7, K = 30;
int ti(char c){
if (c == 'R') return 0;
if (c == 'G') return 1;
return 2;
}
void ckmin(int &a, int b){
a = min(a, b);
}
void solve(){
int n; cin >> n;
string s; cin >> s;
vector<int>cnt(3);
for (auto c: s) cnt[ti(c)]++;
int mx = 0;
for (auto x: cnt) mx = max(mx, x);
int p = 0;
int a = -1, b = -1;
for (int i = 0; i<3; i++) {
if (cnt[i] == mx){
p = i;
}
}
for (int i = 0; i<3; i++){
if (i == p) continue;
if (a == -1) a = i;
else b = i;
}
debug(p, a, b);
s = "$" + s;
vector<vector<int>>pos(3, {-oo});
for (int i = 1; i<=n; i++){
pos[ti(s[i])].emplace_back(i);
}
int dp[n+1][cnt[a]+1][cnt[b]+1][3];
for (int i = 0; i<=n; i++){
for (int A = 0; A <= cnt[a]; A++){
for (int B = 0; B <= cnt[b]; B++){
for (int last = 0; last < 3; last++){
dp[i][A][B][last] = oo;
}
}
}
}
auto ile = [&](int l, int r, int where){
int x = (int)(upper_bound(pos[where].begin(), pos[where].end(), r) - upper_bound(pos[where].begin(), pos[where].end(), l));
return max(0, x);
};
//opt -- szukanie ile() nie zalezy od wartosci i, tylko od A, B, C
//preprocess [now, A], [now, B], [now, C] dla kazdego now \in [1, n] oraz sensownego A, B, C
vector pre(n+1, vector<vector<int>>(3));
for (int now = 1; now <= n; now++){
for (int rep = 0; rep < 3; rep++){
pre[now][rep].resize(cnt[rep]+1);
for (int i = 1; i<=cnt[rep]; i++){
pre[now][rep][i] = ile(now, pos[rep][i], rep);
}
}
}
for (int i = 0; i<3; i++) dp[0][0][0][i] = 0;
for (int i = 1; i<=n; i++){
for (int A = 0; A <= cnt[a]; A++){
for (int B = 0; B <= min(cnt[b], i-A); B++){
int C = i - A - B;
if (C < 0 || C > cnt[p]) continue;
for (int last = 0; last < 3; last++){
for (int prev = 0; prev < 3; prev++){
if (last == prev) continue;
if (last == a){
if (!A) continue;
int now = pos[last][A];
int cost = now-i + pre[now][b][B] + pre[now][p][C];
ckmin(dp[i][A][B][last], dp[i-1][A-1][B][prev] + cost);
} else if (last == b){
if (!B) continue;
int now = pos[last][B];
int cost = now-i + pre[now][a][A] + pre[now][p][C];
ckmin(dp[i][A][B][last], dp[i-1][A][B-1][prev] + cost);
} else {
if (!C) continue;
int now = pos[p][C];
int cost = now-i + pre[now][b][B] + pre[now][a][A];
ckmin(dp[i][A][B][last], dp[i-1][A][B][prev] + cost);
}
}
}
}
}
}
int ans = oo;
for (int last = 0; last < 3; last++){
ans = min(ans, dp[n][cnt[a]][cnt[b]][last]);
}
if (ans == oo) cout << "-1\n";
else cout << ans << "\n";
}
int32_t main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
solve();
return 0;
}
