#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define FOR(i, a, b) for (int i = (a), _b = (b); i <= _b; i++)
#define FORD(i, b, a) for (int i = (b), _a = (a); i >= _a; i--)
#define pa pair<ll, ll>
#define fi first
#define se second
#define bit(mask, j) ((mask >> j) & 1)
const ll mod = 1e9 + 7;
const ll INF = 1e18;
//--------------------------------------------------------------------
ll n;
string T = "RGY";
string s;
const ll N = 402;
ll f[N][N][4][4];
ll dp(ll i, ll j, ll type, ll type2) {
assert(j != 0);
if(i == n) return j == 1 ? 0 : INF;
if(f[i][j][type][type2] != -1) return f[i][j][type][type2];
ll type_new = T.find(s[i]);
if(type_new == type) {
return f[i][j][type][type2] = dp(i + 1, j + 1, type, type2);
}
ll res = INF;
if(type != 3 && type2 != type_new) {
res = min(res, dp(i + 1, j, type, type_new) + j);
}
if(j == 1) {
res = min(res, dp(i + 1, j, type_new, type));
}
else if(j >= 2)
res = min(res, dp(i + 1, j - 1, type, type2) + j - 1);
return f[i][j][type][type2] = res;
}
void hbmt() {
cin >> n;
cin >> s;
memset(f, -1, sizeof f);
ll val = dp(0, 1, 3, 3);
if(val == INF) cout << -1;
else cout << val;
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);cout.tie(0);
#define NAME "hbmt"
if(fopen(NAME".inp", "r")) {
freopen(NAME".inp", "r", stdin);
freopen(NAME".out", "w", stdout);
}
//int t;cin>>t;while(t--)
hbmt();
return 0;
}
