#include<bits/stdc++.h>
using namespace std;
const int MAXN = 405;
const int INF = 1e9;
int n;
string s;
int dp[MAXN][MAXN][MAXN][3];
int pre0[MAXN];
int pre1[MAXN];
int pre2[MAXN];
int pos0[MAXN];
int pos1[MAXN];
int pos2[MAXN];
int cnt0 = 0;
int cnt1 = 0;
int cnt2 = 0;
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> n;
cin >> s;
s = '*' + s;
for (int i = 1; i <= n; i++)
{
pre0[i] = pre0[i - 1];
pre1[i] = pre1[i - 1];
pre2[i] = pre2[i - 1];
if (s[i] == 'R')
{
pre0[i]++;
cnt0++;
pos0[cnt0] = i;
}
else if (s[i] == 'G')
{
pre1[i]++;
cnt1++;
pos1[cnt1] = i;
}
else if (s[i] == 'Y')
{
pre2[i]++;
cnt2++;
pos2[cnt2] = i;
}
}
for (int i = 0; i <= cnt0; i++)
{
for (int j = 0; j <= cnt1; j++)
{
for (int k = 0; k <= cnt2; k++)
{
for (int l = 0; l <= 2; l++)
{
dp[i][j][k][l] = INF;
}
}
}
}
dp[0][0][0][1] = dp[0][0][0][2] = dp[0][0][0][0] = 0;
for (int i = 0; i <= cnt0; i++)
{
for (int j = 0; j <= cnt1; j++)
{
for (int k = 0; k <= cnt2; k++)
{
if (i + 1 <= cnt0)
{
int cur0 = max(0, pre1[pos0[i + 1]] - j) + max(0, pre2[pos0[i + 1]] - k);
dp[i + 1][j][k][0] = min(dp[i + 1][j][k][0], min(dp[i][j][k][1], dp[i][j][k][2]) + cur0);
}
if (j + 1 <= cnt1)
{
int cur1 = max(0, pre0[pos1[j + 1]] - i) + max(0, pre2[pos1[j + 1]] - k);
dp[i][j + 1][k][1] = min(dp[i][j + 1][k][1], min(dp[i][j][k][0], dp[i][j][k][2]) + cur1);
}
if (k + 1 <= cnt2)
{
int cur2 = max(0, pre0[pos2[k + 1]] - i) + max(0, pre1[pos2[k + 1]] - j);
dp[i][j][k + 1][2] = min(dp[i][j][k + 1][2], min(dp[i][j][k][0], dp[i][j][k][1]) + cur2);
}
}
}
}
int ans = INF;
for (int l = 0; l <= 2; l++)
{
ans = min(ans, dp[cnt0][cnt1][cnt2][l]);
}
if (ans == INF)
{
cout << -1;
return 0;
}
cout << ans;
}
