#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define st first
#define nd second
typedef long long ll;
typedef long double ld;
const ll I = 1'000'000'000'000'000'000LL;
const int II = 100'000'000;
const ll M = 1'000'000'007LL;
const int N = 403;
int dp[N][N][N][3], il[N][3], pos[N][3];
int tp[200];
void Solve()
{
tp['R'] = 0; tp['G'] = 1; tp['Y'] = 2;
int n;
string s;
cin >> n >> s;
s = '#' + s;
for(int i = 0; i <= n; ++i)
for(int j = 0; j <= n; ++j)
for(int r = 0; r <= 2; ++r) dp[0][i][j][r] = II;
for(int r = 0; r <= 2; ++r) dp[0][0][0][r] = 0;
for(int l = 1; l <= n; ++l)
{
int a = tp[(int)s[l]];
for(int r = 0; r <= 2; ++r) il[l][r] = il[l - 1][r];
++il[l][a];
pos[il[l][a]][a] = l;
}
for(int l = 1; l <= n; ++l)
{
for(int i = 0; i <= l; ++i)
for(int j = 0; j <= l - i; ++j)
{
int k = l - i - j;
if(i > il[n][0] || j > il[n][1] || k > il[n][2]) continue;
for(int r = 0; r <= 2; ++r)
dp[l][i][j][r] = II;
int p = pos[i][0];
if(i > 0)
dp[l][i][j][0] = min(dp[l - 1][i - 1][j][1], dp[l - 1][i - 1][j][2]) + p + max(0, j - il[p][1]) + max(0, k - il[p][2]) - l;
p = pos[j][1];
if(j > 0)
dp[l][i][j][1] = min(dp[l - 1][i][j - 1][0], dp[l - 1][i][j - 1][2]) + p + max(0, i - il[p][0]) + max(0, k - il[p][2]) - l;
p = pos[k][2];
if(k > 0)
dp[l][i][j][2] = min(dp[l - 1][i][j][0], dp[l - 1][i][j][1]) + p + max(0, j - il[p][1]) + max(0, i - il[p][0]) - l;
//cerr << l << " " << pos[i][0] << " " << dp[l][i][j][0] << " | " << j << " " << pos[j][1] << " " << dp[l][i][j][1] << " | " << k << " " << pos[k][2] << " " << dp[l][i][j][2] << "\n";
}
}
int answer = II;
for(int i = 0; i <= n; ++i)
for(int j = 0; j <= n - i; ++j)
{
int k = n - i - j;
if(i > il[n][0] || j > il[n][1] || k > il[n][2]) continue;
for(int r = 0; r <= 2; ++r)
answer = min(answer, dp[n][i][j][r]);
}
if(answer < II)
cout << answer << "\n";
else
cout << -1 << "\n";
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
//int t; cin >> t;
//while(t--)
Solve();
return 0;
}
