//#define NDEBUG
#include <bits/stdc++.h>
#include <bits/extc++.h>
#define StarBurstStream ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define iter(a) a.begin(), a.end()
#define riter(a) a.rbegin(), a.rend()
#define lsort(a) sort(iter(a))
#define gsort(a) sort(riter(a))
#define mp(a, b) make_pair(a, b)
#define pb(a) push_back(a)
#define eb(a) emplace_back(a)
#define pf(a) push_front(a)
#define pob pop_back()
#define pof pop_front()
#define F first
#define S second
#define printv(a, b) {bool pvaspace=false; \
for(auto pva : a){ \
if(pvaspace) b << " "; pvaspace=true;\
b << pva;\
}\
b << "\n";}
#define pii pair<int, int>
#define pll pair<ll, ll>
#define tiii tuple<int, int, int>
#define mt make_tuple
#define gt(t, i) get<i>(t)
#define iceil(a, b) ((a) / (b) + !!((a) % (b)))
//#define TEST
typedef long long ll;
typedef unsigned long long ull;
using namespace std;
using namespace __gnu_pbds;
const ll MOD = 1000000007;
const ll MAX = 2147483647;
template<typename A, typename B>
ostream &operator<<(ostream &o, pair<A, B> p){
return o << '(' << p.F << ',' << p.S << ')';
}
int main(){
StarBurstStream
int n;
cin >> n;
string s;
cin >> s;
vector<int> rp, gp, yp;
vector<int> rc(n), gc(n), yc(n);
for(int i = 0; i < n; i++){
if(s[i] == 'R') rp.eb(i), rc[i]++;
else if(s[i] == 'G') gp.eb(i), gc[i]++;
else yp.eb(i), yc[i]++;
if(i) rc[i] += rc[i - 1], gc[i] += gc[i - 1], yc[i] += yc[i - 1];
}
vector<vector<ll>> dpr(rp.size() + 1, vector<ll>(gp.size() + 1, MAX));
vector<vector<ll>> dpg(rp.size() + 1, vector<ll>(gp.size() + 1, MAX));
vector<vector<ll>> dpy(rp.size() + 1, vector<ll>(gp.size() + 1, MAX));
dpr[0][0] = 0;
dpg[0][0] = 0;
dpy[0][0] = 0;
for(int i = 1; i <= n; i++){
vector<vector<ll>> dpr2(rp.size() + 1, vector<ll>(gp.size() + 1, MAX));
vector<vector<ll>> dpg2(rp.size() + 1, vector<ll>(gp.size() + 1, MAX));
vector<vector<ll>> dpy2(rp.size() + 1, vector<ll>(gp.size() + 1, MAX));
for(int r = 0; r <= i && r <= rp.size(); r++){
for(int g = 0; r + g <= i && g <= gp.size(); g++){
int y = i - r - g;
if(y > yp.size()) continue;
if(r)
dpr2[r][g] = min(dpg[r - 1][g] + max(0, gc[rp[r - 1]] - g) + max(0, yc[rp[r - 1]] - y),
dpy[r - 1][g] + max(0, gc[rp[r - 1]] - g) + max(0, yc[rp[r - 1]] - y));
if(g)
dpg2[r][g] = min(dpr[r][g - 1] + max(0, yc[gp[g - 1]] - y) + max(0, rc[gp[g - 1]] - r),
dpy[r][g - 1] + max(0, yc[gp[g - 1]] - y) + max(0, rc[gp[g - 1]] - r));
if(y)
dpy2[r][g] = min(dpr[r][g] + max(0, rc[yp[y - 1]] - r) + max(0, gc[yp[y - 1]] - g),
dpg[r][g] + max(0, rc[yp[y - 1]] - r) + max(0, gc[yp[y - 1]] - g));
}
}
dpr = dpr2;
dpg = dpg2;
dpy = dpy2;
}
if(min({dpr[rp.size()][gp.size()], dpg[rp.size()][gp.size()], dpy[rp.size()][gp.size()]}) >= MAX) cout << "-1\n";
else cout << min({dpr[rp.size()][gp.size()], dpg[rp.size()][gp.size()], dpy[rp.size()][gp.size()]}) << "\n";
return 0;
}
