#include <bits/stdc++.h>
using namespace std;
#define finish(x) return cout << x << endl, 0
#define ll long long
const int N = 401;
int n, dp[N][N][N][3], sum[N][3], pos[N][3];
string s;
int get_sum(int idx, int l, int r){
if(r < l || l == -1 || r == -1) return 0;
return sum[r][idx] - (l == 0 ? 0 : sum[l - 1][idx]);
}
bool check(int a, int b, int c){
if(a > c) swap(a, c);
if(b > c) swap(b, c);
if(a > b) swap(a, b);
if(a + b + 1 < c) return 0;
return 1;
}
int solve(int a, int b, int c, int las){
if(!check(a, b, c)) return 1e9;
if(a + b + c == n) return 0;
int &ret = dp[a][b][c][las];
if(ret != -1) return ret;
ret = 1e9;
if((las != 0 || a + b + c == 0) && pos[a][0] != -1){
ret = min(ret, solve(a + 1, b, c, 0) + get_sum(1, pos[b][1], pos[a][0]) + get_sum(2, pos[c][2], pos[a][0]));
}
if((las != 1 || a + b + c == 0) && pos[b][1] != -1){
ret = min(ret, solve(a, b + 1, c, 1) + get_sum(0, pos[a][0], pos[b][1]) + get_sum(2, pos[c][2], pos[b][1]));
}
if((las != 2 || a + b + c == 0) && pos[c][2] != -1){
ret = min(ret, solve(a, b, c + 1, 2) + get_sum(0, pos[a][0], pos[c][2]) + get_sum(1, pos[b][1], pos[c][2]));
}
return ret;
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
memset(pos, -1, sizeof pos);
memset(dp, -1, sizeof dp);
cin >> n >> s;
for(int i = 0 ; i < n ; i++){
if(s[i] == 'R') s[i] = 0;
else if(s[i] == 'G') s[i] = 1;
else s[i] = 2;
for(int j = 0 ; j < 3 ; j++){
if(i > 0) sum[i][j] = sum[i - 1][j];
sum[i][j] += s[i] == j;
}
}
int pt0 = 0, pt1 = 0, pt2 = 0;
for(int i = 0 ; i < n ; i++){
if(s[i] == 0) pos[pt0++][0] = i;
else if(s[i] == 1) pos[pt1++][1] = i;
else pos[pt2++][2] = i;
}
int ans = solve(0, 0, 0, 0);
if(ans == 1e9) ans = -1;
cout << ans << endl;
}
