#include <bits/stdc++.h>
#define task "HOT3"
#define ll long long
#define ld long double
#define fi first
#define se second
#define pb push_back
using namespace std;
const int MAXN = 4e2 + 5;
const ll INF = 1e8 + 5;
int n, dp[MAXN][MAXN][MAXN][3];
string s;
void Input()
{
cin >> n >> s;
s = "," + s;
}
int cnt1[MAXN][3], cnt2[MAXN][3], cnt3[MAXN][3];
void Solve()
{
int cntr = 0, cntg = 0, cnty = 0;
for (int i = 1; i <= n; i++)
{
if (s[i] == 'R')
{
cntr++;
cnt1[cntr][1] = cntg;
cnt1[cntr][2] = cnty;
}
if (s[i] == 'G')
{
cntg++;
cnt2[cntg][0] = cntr;
cnt2[cntg][2] = cnty;
}
if (s[i] == 'Y')
{
cnty++;
cnt3[cnty][0] = cntr;
cnt3[cnty][1] = cntg;
}
}
memset(dp, 44, sizeof(dp));
dp[0][0][0][0] = dp[0][0][0][1] = dp[0][0][0][2] = 0;
for (int i = 0; i <= cntr; i++)
{
for (int j = 0; j <= cntg; j++)
{
for (int z = 0; z <= cnty; z++)
{
if (i < cntr)
dp[i + 1][j][z][0] = min(dp[i + 1][j][z][0], min(dp[i][j][z][1], dp[i][j][z][2]) + abs(cnt1[i + 1][1] - j) + abs(cnt1[i + 1][2] - z));
if (j < cntg)
dp[i][j + 1][z][1] = min(dp[i][j + 1][z][1], min(dp[i][j][z][0], dp[i][j][z][2]) + abs(cnt2[j + 1][0] - i) + abs(cnt2[j + 1][2] - z));
if (z < cnty)
dp[i][j][z + 1][2] = min(dp[i][j][z + 1][2], min(dp[i][j][z][0], dp[i][j][z][1]) + abs(cnt3[z + 1][0] - i) + abs(cnt3[z + 1][1] - j));
}
}
}
int ans = min(dp[cntr][cntg][cnty][0], min(dp[cntr][cntg][cnty][1], dp[cntr][cntg][cnty][2]));
ans /= 2;
cout << (ans < INF ? ans : -1);
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
//freopen(taskname".INP" , "r" , stdin);
//freopen(taskname".OUT" , "w" , stdout);
Input();
Solve();
return 0;
}
