#include <bits/stdc++.h> //#define int long long #define debug cout << "ok\n"; #define SQR(x) (1LL * ((x) * (x))) #define MASK(i) (1LL << (i)) #define BIT(x, i) (((x) >> (i)) & 1) #define fi first #define se second #define pb push_back #define mp make_pair #define pii pair<int,int> #define pli pair<ll,int> #define vi vector<int> #define FAST ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0); typedef long long ll; typedef unsigned long long ull; typedef long double ld; typedef unsigned int ui; using namespace std; const int M = 1e9 + 7; const int INF = 1e9 + 7; const ll INFLL = (ll)2e18 + 7LL; const ld PI = acos(-1); const int dx[] = {1, -1, 0, 0, -1, 1, 1, -1}; const int dy[] = {0, 0, 1, -1, -1, -1, 1, 1}; template<class _, class __> bool minimize(_ &x, const __ y){ if(x > y){ x = y; return true; } else return false; } template<class _, class __> bool maximize(_ &x, const __ y){ if(x < y){ x = y; return true; } else return false; } template<class _,class __> void Add(_ &x, const __ y) { x += y; if (x >= M) { x -= M; } return; } template<class _,class __> void Diff(_ &x, const __ y) { x -= y; if (x < 0) { x += M; } return; } //-------------------------------------------------------------- const int MaxN = 407; int n,f[MaxN][MaxN][MaxN][3],g[3][MaxN][3][MaxN]; vi lis[3]; void sol() { cin >> n; for (int i=0;i<3;i++) lis[i].pb(0); for (int i=1;i<=n;i++) { char x; cin >> x; int xx; if (x == 'R') xx = 0; else if (x == 'G') xx = 1; else xx = 2; lis[xx].pb(i); } for (int i=0;i<3;i++) { for (int j=1;j<lis[i].size();j++) { for (int k=0;k<3;k++) { for (int z=1;z<lis[k].size();z++) { g[i][j][k][z] = g[i][j][k][z-1]; if (lis[k][z] > lis[i][j]) g[i][j][k][z]++; } } } } memset(f,0x3f,sizeof(f)); f[0][0][0][0] = 0; f[0][0][0][1] = 0; f[0][0][0][2] = 0; for (int i=0;i<lis[0].size();i++) { for (int j=0;j<lis[1].size();j++) { for (int k=0;k<lis[2].size();k++) { minimize(f[i+1][j][k][0],f[i][j][k][1] + g[0][i+1][1][j] + g[0][i+1][2][k]); minimize(f[i+1][j][k][0],f[i][j][k][2] + g[0][i+1][1][j] + g[0][i+1][2][k]); minimize(f[i][j+1][k][1],f[i][j][k][0] + g[1][j+1][0][i] + g[1][j+1][2][k]); minimize(f[i][j+1][k][1],f[i][j][k][2] + g[1][j+1][0][i] + g[1][j+1][2][k]); minimize(f[i][j][k+1][2],f[i][j][k][0] + g[2][k+1][0][i] + g[2][k+1][1][j]); minimize(f[i][j][k+1][2],f[i][j][k][1] + g[2][k+1][0][i] + g[2][k+1][1][j]); } } } int res = INF; for (int i=0;i<3;i++) minimize(res,f[(int)lis[0].size()-1][(int)lis[1].size()-1][(int)lis[2].size()-1][i]); if (res >= INF) cout << -1; else cout << res; } signed main() { // freopen("test.inp","r",stdin); // freopen("test.out","w",stdout); FAST int t=1; // cin >> t; while (t--) sol(); }