#include<bits/stdc++.h>
using namespace std;
const int INF = 1e9;
const int MX = 203;
int n;
int dp[MX][MX][MX][3];
vector<int>id[3]; //R G Y
int solve(int r, int g, int y, int prev) {
if(r+g+y==n) return 0;
int &ret = dp[r][g][y][prev];
if(ret!=-1) return ret;
ret = INF;
auto fix = [&] (int &a) {
a = max(0, a);
};
if(r+g+y==0 || prev!=0) {
if(id[0].size()>r) {
int cost1 = upper_bound(id[1].begin(), id[1].end(), id[0][r]) - id[1].begin();
cost1 -= g;
int cost2 = upper_bound(id[2].begin(), id[2].end(), id[0][r]) - id[2].begin();
cost2 -= y;
fix(cost1);
fix(cost2);
ret = min(ret, cost1+cost2+solve(r+1, g, y, 0));
}
}
if(prev!=1 && id[1].size()>g) {
int cost1 = upper_bound(id[0].begin(), id[0].end(), id[1][g]) - id[0].begin();
cost1 -= r;
int cost2 = upper_bound(id[2].begin(), id[2].end(), id[1][g]) - id[2].begin();
cost2 -= y;
fix(cost1);
fix(cost2);
ret = min(ret, cost1+cost2+solve(r, g+1, y, 1));
}
if(prev!=2 && id[2].size()>y) {
int cost1 = upper_bound(id[0].begin(), id[0].end(), id[2][y]) - id[0].begin();
cost1 -= r;
int cost2 = upper_bound(id[1].begin(), id[1].end(), id[2][y]) - id[1].begin();
cost2 -= g;
fix(cost1);
fix(cost2);
ret = min(ret, cost1+cost2+solve(r, g, y+1, 2));
}
return ret;
}
void PlayGround() {
string s;
cin>>n>>s;
for(int i=0; i<n; ++i) {
if(s[i]=='R') id[0].push_back(i);
else if(s[i]=='G') id[1].push_back(i);
else id[2].push_back(i);
}
int mx = max({id[0].size(), id[1].size(), id[2].size()});
if((n+1)/2<mx) {
cout<<"-1\n";
return;
}
memset(dp, -1, sizeof dp);
cout<<solve(0, 0, 0, 0)<<'\n';
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
PlayGround();
return 0;
}
