/***Farhan132***/
//#pragma GCC optimize("Ofast")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,fma")
//#pragma GCC optimization ("unroll-loops")
//#pragma GCC optimize("fast-math")
#include <bits/stdc++.h>
using namespace std;
typedef int ll;
typedef unsigned long long ull;
typedef double dd;
typedef vector<ll> vll;
typedef pair<ll , ll> ii;
typedef vector< ii > vii;
typedef pair < pair < ll , ll > , pair < ll , ll > > cm;
typedef tuple < ll, ll, ll > tp;
#define ff first
#define ss second
#define pb push_back
#define in insert
#define f0(b) for(int i=0;i<(b);i++)
#define f00(b) for(int j=0;j<(b);j++)
#define f1(b) for(int i=1;i<=(b);i++)
#define f11(b) for(int j=1;j<=(b);j++)
#define f2(a,b) for(int i=(a);i<=(b);i++)
#define f22(a,b) for(int j=(a);j<=(b);j++)
#define sf(a) scanf("%lld",&a)
#define sff(a,b) scanf("%lld %lld", &a , &b)
#define pf(a) printf("%lld\n",a)
#define pff(a,b) printf("%lld %lld\n", a , b)
#define bug printf("**!\n")
#define mem(a , b) memset(a, b ,sizeof(a))
#define front_zero(n) __builtin_clzll(n)
#define back_zero(n) __builtin_ctzll(n)
#define total_one(n) __builtin_popcount(n)
#define clean fflush(stdout)
//const ll mod = (ll) 998244353;
const ll mod = (ll) 1e9 + 7;
const ll maxn = (ll)1e8 + 5;
const int nnn = 1048590;
//const int inf = numeric_limits<int>::max()-1;
const int inf = 1e9;
//const ll INF = numeric_limits<ll>::max()-1;
const ll INF = (ll)1e18;
ll dx[]={0,1,0,-1};
ll dy[]={1,0,-1,0};
ll dxx[]={0,1,0,-1,1,1,-1,-1};
ll dyy[]={1,0,-1,0,1,-1,1,-1};
bool USACO = 0;
mt19937 rng(chrono::system_clock::now().time_since_epoch().count());
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
template <typename T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
const ll N = 405;
ll dp[N][N][N][3];
ll pre[3][N];
vector < ll > v[N];
void solve(){
ll n; cin >> n;
string s; cin >> s;
mem(pre, 0);
for(ll i = 0; i < n; i++){
if(s[i] == 'R') v[0].pb(i + 1), pre[0][i+1]++;
if(s[i] == 'G') v[1].pb(i + 1), pre[1][i+1]++;
if(s[i] == 'Y') v[2].pb(i + 1), pre[2][i+1]++;
for(ll j = 0; j < 3; j++) pre[j][i + 1] += pre[j][i];
}
char a[] = {'R', 'G', 'Y'};
ll ans = inf;
dp[0][0][0][0] = dp[0][0][0][1] = dp[0][0][0][2] = 0;
for(ll i = 0; i <= v[0].size(); i++){
for(ll j = 0; j <= v[1].size(); j++){
for(ll k = 0; k <= v[2].size(); k++){
if(max({i, j, k}) == 0) continue;
for(ll now = 0; now <= 2; now++){
dp[i][j][k][now] = inf;
vector < ll > c(3, 0);
vector < ll > w = {i-1, j-1, k-1};
c[now] = -1;
if(min({i + c[0], j + c[1], k + c[2]}) < 0) continue;
for(ll last = 0; last <= 2; last++){
if(now == last) continue;
ll b = v[now][w[now]];
ll vv = b;
for(ll t = 0; t < 3; t++){
if(w[t] >= 0) vv -= pre[t][min(b, v[t][w[t]])];
}
dp[i][j][k][now] = min(dp[i][j][k][now] , dp[i + c[0]][j + c[1]][k + c[2]][last] + vv);
}
if(i + j + k == n) ans = min(dp[i][j][k][now] , ans);
}
}
}
}
if(ans == inf) ans = -1;
cout << ans << '\n';
return;
}
int main() {
#ifdef LOCAL
freopen("in", "r", stdin);
freopen("out", "w", stdout);
auto start_time = clock();
#else
ios_base::sync_with_stdio(0); cin.tie(NULL); cout.tie(NULL);
#endif
ll T = 1, CT = 0; //cin >> T;
while(T--){
solve();
}
#ifdef LOCAL
auto end_time = clock();
cerr<< "Execution time: "<<(end_time - start_time)*(int)1e3/CLOCKS_PER_SEC<<" ms\n";
#endif
return 0;
}
