#include <bits/stdc++.h>
using namespace std;
#define mod 1000000007
#define mp make_pair
#define pb push_back
#define bas(x) #x << ": " << x << " "
#define prarr(x, n) cout << #x << ": "; for (int qsd = 0; qsd < n; qsd++) cout << x[qsd] << " "; cout << endl;
#define prarrv(x) cout << #x << ": "; for (int qsd = 0; qsd < (int)x.size(); qsd++) cout << x[qsd] << " "; cout << endl;
#define ppair(x) "(" << x.first << ", " << x.second << ")"
#define inside sl<=l&&r<=sr
#define outside sr<l||r<sl
typedef long long ll;
string s;
int dist(string str){
string ana = s;
ll ans = 0;
int say = 0;
for (int i = str.size()-1; i >= 0; i--){
for (int j = ana.size()-1; j >= 0; j--){
if (ana[j] == str[i]){
ana[j] = 'x';
say++;
break;
} else if (ana[j] != 'x') ans++;
}
}
if (say != str.size()) return 1000000000;
return ans;
}
int dp[404][404][404][4];
vector<int> rler, gler, yler;
int sufsumg[404];
int sufsumy[404];
int sufsumr[404];
int f(int r, int g, int y, int last){
//cout << "f(" << r << ", " << g << ", " << y << ", " << last << ")" << endl;
if (dp[r][g][y][last] != -1) return dp[r][g][y][last];
int minn = 400*400+2;
if ((r > 0) && (last != 0)){
int ret = f(r-1, g, y, 0);
int ind = s.size() -r-g-y;
ret += max(sufsumr[ind]-1, r-1) + max(sufsumg[ind], g) + max(sufsumy[ind], y) - (g+y+r-1);
minn = min(minn, ret);
}
if (g > 0 && last != 1){
int ret = f(r, g-1, y, 1);
int ind = s.size() -r-g-y;
ret += max(sufsumr[ind], r) + max(sufsumg[ind]-1, g-1) + max(sufsumy[ind], y) - (g+y+r-1);
minn = min(minn, ret);
}
if (y > 0 && last != 2){
int ret = f(r, g, y-1, 2);
int ind = s.size() -r-g-y;
ret += max(sufsumr[ind], r) + max(sufsumg[ind], g) + max(sufsumy[ind]-1, y-1) - (g+y+r-1);
minn = min(minn, ret);
}
if (g == 0 && r == 0 && y == 0){
return dp[r][g][y][last] = 0;
}
return dp[r][g][y][last] = minn;
}
int main(){
//freopen("in", "r", stdin);
//freopen("out", "w", stdout);
clock_t start = clock();
int n;
cin >> n;
cin >> s;
n = s.size();
int r = 0, g = 0, y = 0;
for (int i = 0; i < n; i++){
r += s[i] == 'R';
g += s[i] == 'G';
y += s[i] == 'Y';
if (s[i] == 'R') rler.pb(i);
else if (s[i] == 'G') gler.pb(i);
else yler.pb(i);
}
for (int i1 = 0; i1 <= r; i1++)
for (int i2 = 0; i2 <= g; i2++)
for (int i3 = 0; i3 <= y; i3++)
for (int i4 = 0; i4 < 4; i4++)
dp[i1][i2][i3][i4] = -1;
int sufr = 0, sufg = 0, sufy = 0;
for (int i = n-1; i >= 0; i--){
if (s[i] == 'R') sufr++;
if (s[i] == 'G') sufg++;
if (s[i] == 'Y') sufy++;
sufsumr[i] = sufr;
sufsumg[i] = sufg;
sufsumy[i] = sufy;
}
//cout << f(1, 0, 0, 1) << endl;
int ans = f(r, g, y, 3);
//int ans = 0;
if (ans >= 400*400+2) cout << -1 << endl;
else cout << ans << endl;
//cout << "bitti" << endl;
//cout << 1000.0*(clock()-start)/(double)CLOCKS_PER_SEC << "ms" << endl;
}
