#include <bits/stdc++.h>
using namespace std;
//#define int long long
#define F first
#define S second
#define pii pair<int, int>
#define ppp pair<int, pii>
const int maxn = 1e6 + 10, maxm = 4e2 + 1, oo = 1e9;
int n, a[maxm], dp[maxm][maxm][maxm][3], c[3], ind[3][maxm], pre[3][maxm];
signed main(){
cin >> n;
string st;
cin >> st;
st = "a" + st;
for (int i = 1; i <= n; i++){
if(st[i] == 'R')
a[i] = 0;
else if(st[i] == 'G')
a[i] = 1;
else
a[i] = 2;
ind[a[i]][++c[a[i]]] = i;
for (int j = 0; j < 3;j++)
pre[j][i] = c[j];
}
for (int i = 0; i <= n;i++)
for (int j = 0; j <= c[0];j++)
for (int k = 0; k <= c[1];k++)
for (int l = 0; l < 3;l++)
dp[i][j][k][l] = oo;
for (int l = 0; l < 3;l++)
dp[0][0][0][l] = 0;
for (int i = 1; i <= n;i++)
for (int r = 0; r <= min(i, c[0]);r++)
for (int g = 0; g <= min(i - r, c[1]);g++){
int b = i - r - g;
if(b > c[2])
continue;
if(r > 0)
dp[i][r][g][0] = min(dp[i - 1][r - 1][g][1], dp[i - 1][r - 1][g][2]) + max(0, g - pre[1][ind[0][r]]) + max(0, b - pre[2][ind[0][r]]);
if(g > 0)
dp[i][r][g][1] = min(dp[i - 1][r][g - 1][0], dp[i - 1][r][g - 1][2]) + max(0, r - pre[0][ind[1][g]]) + max(0, b - pre[2][ind[1][g]]);
if(b > 0)
dp[i][r][g][2] = min(dp[i - 1][r][g][0], dp[i - 1][r][g][1]) + max(0, r - pre[0][ind[2][b]]) + max(0, g - pre[1][ind[2][b]]);
}
int ans = min({dp[n][c[0]][c[1]][0], dp[n][c[0]][c[1]][1], dp[n][c[0]][c[1]][2]});
cout << (ans >= oo ? -1 : ans);
}
