# include <bits/stdc++.h>
using namespace std;
int n,pos[405][5],cnt[5],dp[405][405][405][3],pref[405][3];
string s;
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
cin >> n;
cin >> s;
s = '#' + s;
for (int i =1; i <= n; i++)
{
if (s[i]=='R')
{
cnt[0]++;
pos[cnt[0]][0] = i;
pref[i][0] ++;
}
else if (s[i]=='G')
{
cnt[1] ++;
pos[cnt[1]][1]=i;
pref[i][1]++;
}
else
{
cnt[2]++;
pos[cnt[2]][2]= i;
pref[i][2]++;
}
pref[i][0] += pref[i-1][0];
pref[i][1] += pref[i-1][1];
pref[i][2] += pref[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 p = 0; p <= 2; p++)dp[i][j][k][p]=1e9;
dp[0][0][0][0] = 0;
dp[0][0][0][1]=0;
dp[0][0][0][2] = 0;
for (int cnt0 = 0; cnt0 <= pref[n][0]; cnt0++)
{
for (int cnt1 = 0; cnt1 <= pref[n][1]; cnt1++)
{
for (int cnt2 = 0; cnt2 <= pref[n][2]; cnt2++)
{
for (int color = 0; color < 3; color++)
{
if (dp[cnt0][cnt1][cnt2][color] != 1e9)
{
for (int ncolor = 0; ncolor < 3; ncolor++)
{
if (ncolor==color) continue;
int conlai = 3-color-ncolor;
int b[3] = {cnt0,cnt1,cnt2};
b[ncolor]++;
int end = pos[b[ncolor]][ncolor];
dp[b[0]][b[1]][b[2]][ncolor]= min(dp[b[0]][b[1]][b[2]][ncolor],dp[cnt0][cnt1][cnt2][color] + end-1 - min(cnt0, pref[end][0]) - min(cnt1,pref[end][1])-min(cnt2,pref[end][2]));
}
}
}
}
}
}
int res = min({dp[pref[n][0]][pref[n][1]][pref[n][2]][0],dp[pref[n][0]][pref[n][1]][pref[n][2]][1],dp[pref[n][0]][pref[n][1]][pref[n][2]][2]});
if (res==1e9)
{
cout << -1;
}
else cout << res;
}
