#include <bits/stdc++.h> using namespace std; using ll = long long; #include <ext/pb_ds/assoc_container.hpp> #include <ext/pb_ds/tree_policy.hpp> using namespace __gnu_pbds; template<class T> using o_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>; int main() { ios_base::sync_with_stdio(false); cin.tie(NULL); ll n; cin >> n; string s; cin >> s; ll r = 0, g = 0, y = 0; o_set<ll> R; o_set<ll> G; o_set<ll> Y; for(ll j = 0; j < n; j++){ char i = s[j]; if(i == 'R'){ r++; R.insert(j); }else if(i == 'G'){ g++; G.insert(j); }else if(i == 'Y'){ y++; Y.insert(j); } } if(max(max(r, g), y) > (n + 1)/2){ cout << -1 << endl; return 0; } if(n == 1){ cout << 0 << endl; return 0; } //ll dp[n + 5][n + 5][n + 5][3]; // {0, 1, 2} = {R, G, Y} vector<vector<vector<vector<ll>>>> dp(n + 5LL, vector<vector<vector<ll>>>(n + 5LL, vector<vector<ll>>(n + 5LL, {(ll)1e17, (ll)1e17, (ll)1e17}))); dp[0][0][0][0] = 0; dp[0][0][0][1] = 0; dp[0][0][0][2] = 0; for(ll i = 1; i <= n; i++){ for(ll number_r = 0; number_r <= min(i, r); number_r++){ for(ll number_g = 0; number_g <= min(i - number_r, g); number_g++){ ll number_y = i - number_g - number_r; if(number_y > y){ continue; } if(number_r > 0){ ll prev = min(dp[number_r - 1][number_g][number_y][1], dp[number_r - 1][number_g][number_y][2]); ll t = *R.find_by_order(number_r - 1LL); ll temp = t; temp -= min((ll)(G.order_of_key(t)), number_g); temp -= min((ll)(Y.order_of_key(t)), number_y); temp -= number_r - 1; dp[number_r][number_g][number_y][0] = min(prev + temp, dp[number_r][number_g][number_y][0]); } if(number_g > 0){ ll prev = min(dp[number_r][number_g - 1][number_y][0], dp[number_r][number_g - 1][number_y][2]); ll t = *G.find_by_order(number_g - 1LL); ll temp = t; temp -= min((ll)(R.order_of_key(t)), number_r); temp -= min((ll)(Y.order_of_key(t)), number_y); temp -= number_g - 1; dp[number_r][number_g][number_y][1] = min(prev + temp, dp[number_r][number_g][number_y][1]); } if(number_y > 0){ ll prev = min(dp[number_r][number_g][number_y - 1][0], dp[number_r][number_g][number_y - 1][1]); ll t = *Y.find_by_order(number_y - 1LL); ll temp = t; temp -= min((ll)(R.order_of_key(t)), number_r); temp -= min((ll)(G.order_of_key(t)), number_g); temp -= number_y - 1; dp[number_r][number_g][number_y][2] = min(prev + temp, dp[number_r][number_g][number_y][2]); } } } } ll ans = 1e17; for(auto i : dp[r][g][y]){ ans = min(i, ans); } cout << ans; cout << endl; }