#include <bits/stdc++.h>
#define ll long long
#define fi first
#define se second
#define pii pair <int , int>
#define arr3 array <int , 3>
using namespace std;
const int INF = 1e9;
const int maxn = 4e2 + 2;
char s[maxn];
int n , dp[maxn][maxn][maxn][3] , cntr[maxn] , cntg[maxn] , cntb[maxn];
vector <int> posr = {-1} , posg = {-1} , posb = {-1};
void solve()
{
cin >> n;
for(int i = 1; i <= n; i++) cin >> s[i];
for(int i = 1; i <= n; i++)
{
cntr[i] = cntr[i-1];
cntg[i] = cntg[i-1];
cntb[i] = cntb[i-1];
if(s[i] == 'R')
{
posr.push_back(i);
cntr[i]++;
}
else if(s[i] == 'G')
{
posg.push_back(i);
cntg[i]++;
}
else // B
{
posb.push_back(i);
cntb[i]++;
}
if(max({cntr[i] , cntg[i] , cntb[i]}) > (n+1)/2)
{
cout << -1 << '\n';
return;
}
}
for(int r = 0; r < posr.size(); r++)
{
for(int g = 0; g < posg.size(); g++)
{
for(int b = 0; b < posb.size(); b++)
{
if(r == 0 && g == 0 && b == 0) continue;
// case R - 0 case
if(r > 0)
{
dp[r][g][b][0] = min(dp[r-1][g][b][1] , dp[r-1][g][b][2]) + max(0 , g - cntg[posr[r]]) + max(0 , b - cntb[posr[r]]);
}
else dp[r][g][b][0] = INF;
// case G - 1 case
if(g > 0)
{
dp[r][g][b][1] = min(dp[r][g-1][b][0] , dp[r][g-1][b][2]) + max(0 , r - cntr[posg[g]]) + max(0 , b - cntb[posg[g]]);
}
else dp[r][g][b][1] = INF;
// case B - 2 case
if(b > 0)
{
dp[r][g][b][2] = min(dp[r][g][b-1][0] , dp[r][g][b-1][1]) + max(0 , r - cntr[posb[b]]) + max(0 , g - cntg[posb[b]]);
}
else dp[r][g][b][2] = INF;
}
}
}
int ans = INF;
for(int i = 0; i < 3; i++)
{
ans = min(dp[posr.size() - 1][posg.size() - 1][posb.size() - 1][i] , ans);
}
cout << ans << '\n';
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
solve();
return 0;
}
