#include <algorithm>
#include <iostream>
#include <vector>
typedef long long llong;
const int MAXN = 400 + 10;
const int INF = 1e9;
int n;
char s[MAXN];
int dp[MAXN][MAXN][MAXN][4];
bool bl[MAXN][MAXN][MAXN][4];
int byPos[4][MAXN];
int len[4];
int binary(int a[], int bound, int border)
{
int l = -1, r = bound, mid;
while (l < r - 1)
{
mid = (l + r) / 2;
if (a[mid] <= border) l = mid;
else r = mid;
}
return bound - 1 - l;
}
int f(int ones, int twos, int threes, int last)
{
if (ones + twos + threes == n) return 0;
if (bl[ones][twos][threes][last]) return dp[ones][twos][threes][last];
bl[ones][twos][threes][last] = true;
dp[ones][twos][threes][last] = INF;
if (last != 1 && ones != len[1])
{
int add = binary(byPos[2], twos, byPos[1][ones]) + binary(byPos[3], threes, byPos[1][ones]);
dp[ones][twos][threes][last] = std::min(dp[ones][twos][threes][last], f(ones + 1, twos, threes, 1) + add + byPos[1][ones] - (ones + twos + threes + 1));
}
if (last != 2 && twos != len[2])
{
int add = binary(byPos[1], ones, byPos[2][twos]) + binary(byPos[3], threes, byPos[2][twos]);
dp[ones][twos][threes][last] = std::min(dp[ones][twos][threes][last], f(ones, twos + 1, threes, 2) + add + byPos[2][twos] - (ones + twos + threes + 1));
}
if (last != 3 && threes != len[3])
{
int add = binary(byPos[1], ones, byPos[3][threes]) + binary(byPos[2], twos, byPos[3][threes]);
dp[ones][twos][threes][last] = std::min(dp[ones][twos][threes][last], f(ones, twos, threes + 1, 3) + add + byPos[3][threes] - (ones + twos + threes + 1));
}
return dp[ones][twos][threes][last];
}
void solve()
{
for (int i = 1 ; i <= n ; ++i)
{
if (s[i] == 'R') s[i] = '1';
if (s[i] == 'G') s[i] = '2';
if (s[i] == 'Y') s[i] = '3';
byPos[s[i]-'0'][len[s[i]-'0']++] = i;
}
int result = f(0, 0, 0, 0);
if (result == INF) result = -1;
std::cout << result << '\n';
}
void read()
{
std::cin >> n;
std::cin >> s + 1;
}
void fastIO()
{
std::ios_base :: sync_with_stdio(0);
std::cout.tie(nullptr);
std::cin.tie(nullptr);
}
int main ()
{
fastIO();
read();
solve();
return 0;
}
