#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
#define rep(a,b) for(int a = 0; a < (b); ++a)
#define all(t) t.begin(), t.end()
#define pb push_back
const int N = 75, INF = 2e9+54321;
const ll INF_L = (ll)2e18+54321;
int n;
string A;
int B[N];
int S[3][N];
vector<int> V[3];
int dp[N][N][N][3];
void solve()
{
cin >> n >> A;
rep(i,n)
{
if(A[i] == 'R') B[i] = 0;
else if(A[i] == 'Y') B[i] = 1;
else B[i] = 2;
}
rep(i,3)
{
int s = 0;
rep(j,n)
{
S[i][j] = s;
if(B[j] == i) ++s;
}
}
rep(i,n) V[B[i]].pb(i);
rep(i,N) rep(j,N) rep(k,N) rep(f,3) dp[i][j][k][f] = INF;
if((int)V[0].size() > 0) dp[1][1][0][0] = V[0][0];
if((int)V[1].size() > 0) dp[1][0][1][1] = V[1][0];
if((int)V[2].size() > 0) dp[1][0][0][2] = V[2][0];
for(int i = 1; i < n; ++i)
{
for(int A = 0; A <= n; ++A)
{
for(int B = 0; B <= n; ++B)
{
if(n-A-B < 0) continue;
int C = i-A-B;
int W[3];
W[0] = A, W[1] = B, W[2] = C;
rep(k,3)
{
if(dp[i][A][B][k] >= INF) continue;
rep(f,3)
{
if(k == f or (int)V[f].size() <= W[f]) continue;
int idx = V[f][W[f]], poz = 0;
poz = idx + (max(0,W[0]-S[0][idx])) + (max(0,W[1]-S[1][idx])) + (max(0,W[2]-S[2][idx]));
//cout << "A: " << A << " B: " << B << " C: " << C << " K: " << k << " F: " << f << " POZ: " << poz << '\n';
int H[3];
H[0] = W[0], H[1] = W[1], H[2] = W[2], ++H[f];
int cost = dp[i][W[0]][W[1]][k]+(poz-i);
//cout << '\n';
//cout << "I: " << i << " A: " << A << " B: " << B << " C: " << C << " K: " << k << " F: " << f << " POZ: " << poz << " COST: " << cost << '\n';
//cout << "H[0]: " << H[0] << " H[1]: " << H[1] << " H[2]: " << H[2] << '\n';
//cout << '\n';
dp[i+1][H[0]][H[1]][f] = min(dp[i+1][H[0]][H[1]][f],cost);
}
}
}
}
}
int wyn = INF;
rep(i,N) rep(j,N) rep(k,3) wyn = min(wyn,dp[n][i][j][k]);
if(wyn >= INF) cout << "-1" << '\n';
else cout << wyn << '\n';
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
int T = 1;
//cin >> T;
while(T--) solve();
return 0;
}
