#include <bits/stdc++.h>
const int MAXN = 405;
using namespace std;
template<class X, class Y> bool minimize(X &x, const Y &y){return x > y ? x = y, 1: 0;}
template<class X, class Y> bool maximize(X &x, const Y &y){return x < y ? x = y, 1: 0;}
int lenS, s[MAXN], dp[3][MAXN][MAXN][MAXN], pre[3][MAXN], pos[3][MAXN], freq[3], infINT = 1e9 + 712823;
int id(const char &c){
if (c == 'R') return 0;
if (c == 'G') return 1;
return 2;
}
void input(){
cin >> lenS;
for(int i = 1; i <= lenS; i++){
char c; cin >> c;
s[i] = id(c);
}
}
void init(){
for(int i = 1; i <= lenS; i++){
freq[s[i]]++;
pos[s[i]][freq[s[i]]] = i;
for(int type = 0; type < 3; type++)
pre[type][i] = freq[type];
}
}
int get(int c1, int c2){
return c1 ^ c2 ^ 1 ^ 2 ^ 0;
}
int getRange(int type, int L, int R){
if (L > R) return 0;
return pre[type][R] - pre[type][L];
}
void solve(){
init();
memset(dp, 0x3f, sizeof dp);
for(int i = 0; i < 3; i++) dp[i][0][0][0] = 0;
for(int i = 0; i <= freq[0]; i++)
for(int j = 0; j <= freq[1]; j++)
for(int k = 0; k <= freq[2]; k++)
for(int last = 0; last < 3; last++) if (dp[last][i][j][k] < infINT)
for(int nxt = 0; nxt < 3; nxt++) if (last != nxt){
int other = get(last, nxt);
int idLast = (last == 0 ? i: (last == 1 ? j: k));
int idNxt = (nxt == 0 ? i: (nxt == 1 ? j: k)) + 1;
int idOther = (other == 0 ? i: (other == 1 ? j: k));
int posLast = pos[last][idLast];
int posNxt = pos[nxt][idNxt];
int posOther = pos[other][idOther];
int cost = getRange(last, posLast, posNxt) + getRange(other, posOther, posNxt);
minimize(dp[nxt][i + (nxt == 0)][j + (nxt == 1)][k + (nxt == 2)], dp[last][i][j][k] + cost);
}
int res = infINT;
for(int i = 0; i < 3; i++) minimize(res, dp[i][freq[0]][freq[1]][freq[2]]);
cout << (res < infINT ? res: -1) << '\n';
}
int main(){
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
input();
solve();
}
