#include<bits/stdc++.h>
#define IOS ios_base::sync_with_stdio(false);cin.tie();cout.tie();
#define all(x) x.begin(), x.end()
#define int long long
#define pq priority_queue
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define pb push_back
#define pp pop_back
#define F first
#define S second
using namespace std;
const int mxN=400;
int n, dp[410][8][410][4], b[410][7];
string s;
void solve () {
cin >> n >> s;
for (char &c : s) c = (c > 'G') + (c > 'R');
for (int i = 0; i < n; i++) {
if (i) memcpy(b[i], b[i-1], sizeof(b[0]));
for (int j = 1; j < 7; j++)
if (j >> s[i] & 1) b[i][j] = s[i];
}
memset(dp, 0x3f, sizeof(dp));
dp[0][1<<s[0]][1][3] = 0;
auto tm = [](int &a, int b) {a = min(a, b);};
for (int i = 1; i < n; i++)
for (int j = 1; j < 7; j++)
for (int k = 1; k <= i; k++)
for (int l = 0; l < 4; l++) {
tm(dp[i][j|1<<s[i]][k+1][l], dp[i-1][j][k][l]);
if (j >> s[i] & 1) continue ;
if (l != s[i]) tm(dp[i][j][k][s[i]], dp[i-1][j][k][l]+k);
if (k > 1) tm(dp[i][j][k-1][s[i]], dp[i-1][j][k][l]+k-1);
else tm(dp[i][1<<s[i]][1][b[i-1][j]], dp[i-1][j][1][l]);
}
int ans = 1e9;
for (int i = 1; i < 5; i++)
for (int j = 0; j < 4; j++)
ans = min(dp[n-1][i][1][j], ans);
cout << (ans >= 1e9 ? -1 : ans);
}
signed main() {IOS solve(); return 0;}
