#include<iostream>
#include<algorithm>
#include<utility>
#include<vector>
#include<cmath>
using namespace std;
typedef long long ll;
int dp[405][405][405][3];
const int inf = 1e9;
int main(){
ios::sync_with_stdio(false);
cin.tie(0);
int n;
cin >> n;
string s;
cin >> s;
vector<vector<int>> cols(3, vector<int>(1)), pre(n + 1, vector<int>(3));
int sr = 0, sg = 0, sb = 0;
for(int i = 0; i < n; i++){
pre[i + 1][0] = pre[i][0];
pre[i + 1][1] = pre[i][1];
pre[i + 1][2] = pre[i][2];
if(s[i] == 'R'){
cols[0].push_back(i + 1);
sr++;
pre[i + 1][0]++;
}
else if(s[i] == 'G'){
cols[1].push_back(i + 1);
sg++;
pre[i + 1][1]++;
}
else{
cols[2].push_back(i + 1);
sb++;
pre[i + 1][2]++;
}
}
for(int i = 0; i <= n; i++) for(int j = 0; j <= n; j++) for(int k = 0; k <= n; k++) for(int l = 0; l < 3; l++) dp[i][j][k][l] = inf;
dp[0][0][0][0] = 0;
dp[0][0][0][1] = 0;
dp[0][0][0][2] = 0;
auto mabs = [&](int x){
return max(x, 0);
};
for(int i = 1; i <= n; i++){
for(int cr = 0; cr <= sr; cr++){
for(int cg = 0; cg <= sg; cg++){
for(int col = 0; col < 3; col++){
if(i - cr - cg < 0 || i - cr - cg > sb) continue;
if(cr == 0 && col == 0) continue;
if(cg == 0 && col == 1) continue;
if(i - cr - cg == 0 && col == 2) continue;
int cost, cb = i - cr - cg;
if(col == 0) cost = mabs(pre[cols[0][cr]][1] - cg) + mabs(pre[cols[0][cr]][2] - cb);
else if(col == 1) cost = mabs(pre[cols[1][cg]][0] - cr) + mabs(pre[cols[1][cg]][2] - cb);
else cost = mabs(pre[cols[2][cb]][0] - cr) + mabs(pre[cols[2][cb]][1] - cg);
int dpc = inf;
for(int k = 0; k < 3; k++){
if(k != col){
if(col == 0) dpc = min(dpc, dp[i - 1][cr - 1][cg][k] + cost);
else if(col == 1) dpc = min(dpc, dp[i - 1][cr][cg - 1][k] + cost);
else dpc = min(dpc, dp[i - 1][cr][cg][k] + cost);
}
}
dp[i][cr][cg][col] = dpc;
}
}
}
}
int ans = inf;
for(int i = 0; i < 3; i++) ans = min(ans, dp[n][sr][sg][i]);
if(ans == inf) cout << "-1\n";
else cout << ans << "\n";
}
// g++ -std=c++17 -Wall -Wextra -Wshadow -fsanitize=undefined -fsanitize=address -o run pC.cpp
