#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <bits/stdc++.h>
#define fi first
#define se second
#define pb push_back
#define sz(a) (int)a.size()
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define ff(i,a,b) for(int i=a;i<=b;i++)
#define fb(i,b,a) for(int i=b;i>=a;i--)
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef pair<int,int> pii;
const int maxn = 405;
const int inf = 1e9 + 5;
template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
// os.order_of_key(k) the number of elements in the os less than k
// *os.find_by_order(k) print the k-th smallest number in os(0-based)
int n;
int niz[maxn];
string s;
int dp[maxn][maxn][maxn][3]; // 0 - najmanji broj operacija, 1 - zadnja boja
int cnt[maxn][3];
vector<int> poz[maxn];
int f(char C){
if(C == 'R')return 0;
if(C == 'G')return 1;
if(C == 'Y')return 2;
assert(false);
}
int main()
{
ios::sync_with_stdio(false);
cout.tie(nullptr);
cin.tie(nullptr);
cin >> n >> s;
ff(i,0,n - 1)niz[i + 1] = f(s[i]);
ff(i,1,n){
ff(j,0,2)cnt[i][j] = cnt[i - 1][j];
cnt[i][niz[i]] += 1;
poz[niz[i]].pb(i);
}
if(max({cnt[n][0], cnt[n][1], cnt[n][2]}) > (n + 1) / 2)return cout << -1, 0;
int R = cnt[n][0], G = cnt[n][1], Y = cnt[n][2];
ff(a,0,R)ff(b,0,G)ff(c,0,Y)ff(i,0,2)dp[a][b][c][i] = inf;
dp[0][0][0][0] = dp[0][0][0][1] = dp[0][0][0][2] = 0;
ff(a,0,R){
ff(b,0,G){
ff(c,0,Y){
ff(i,0,2){
if(a > 0 && i != 0){
// stavaljam R
int kol = dp[a - 1][b][c][i];
kol += max(0, cnt[poz[0][a - 1]][1] - b);
kol += max(0, cnt[poz[0][a - 1]][2] - c);
dp[a][b][c][0] = min(dp[a][b][c][0], kol);
}
if(b > 0 && i != 1){
int kol = dp[a][b - 1][c][i];
kol += max(0, cnt[poz[1][b - 1]][0] - a);
kol += max(0, cnt[poz[1][b - 1]][2] - c);
dp[a][b][c][1] = min(dp[a][b][c][1], kol);
}
if(c > 0 && i != 2){
int kol = dp[a][b][c - 1][i];
kol += max(0, cnt[poz[2][c - 1]][0] - a);
kol += max(0, cnt[poz[2][c - 1]][1] - b);
dp[a][b][c][2] = min(dp[a][b][c][2], kol);
}
}
}
}
}
int rez = inf;
ff(i,0,2)rez = min(rez, dp[R][G][Y][i]);
cout << rez << endl;
return 0;
}
/**
6
RRGGYY
2
RG
20
YYGYYYGGGGRGYYGRGRYG
5
RRGYY
// probati bojenje sahovski ili slicno
**/
