/*input
5
RRGYY
*/
#pragma GCC optimize ("O3")
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
unordered_map<bitset<1200>, int>M[3];
bitset<1200>xx[400];
int dp(bitset<1200>s, int x, int n)
{
if (M[x].count(s))
return M[x][s];
if (n == 1)
{
for (int i = 0; i < 400; i++)
if (s[400 * x + i])
return M[x][s] = 0;
return M[x][s] = 1e8 + 5;
}
int i = s._Find_next(x * 400 - 1) - 400 * x;
if (i < 400)
{
int cost = (s & xx[i]).count() - 1;
bitset<1200>ss = s;
ss[x * 400 + i] = false;
int mini = 3e8;
for (int y = 0; y <= 2; y++)
{
if (x != y)
{
mini = min(mini, dp(ss, y, n - 1));
}
}
return M[x][s] = cost + mini;
}
return M[x][s] = 3e8;
}
int code(char c)
{
if (c == 'R')
return 0;
if (c == 'G')
return 1;
return 2;
}
int main()
{
bitset<1200>aa;
for (int i = 0; i < 400; i++)
{
aa[i] = aa[400 + i] = aa[800 + i] = true;
xx[i] = aa;
}
ios_base::sync_with_stdio(false);
int n;
string s;
cin >> n >> s;
bitset<1200>ss;
for (int i = 0; i < n; i++)
{
ss[400 * code(s[i]) + i] = true;
}
int ans = min({dp(ss, 0, n), dp(ss, 1, n), dp(ss, 2, n)});
if (ans > 1e8)
ans = -1;
cout << ans << "\n";
}
