#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define pii pair<int, int>
#define pb push_back
#define ff first
#define ss second
const int INF = 1e9 + 10;
int dp[401][401][401][3];
int a[401];
int cnt[401][3];
vector<int> q[3];
void solve(){
int n;
cin>>n;
int cnt0 = 0, cnt1 = 0, cnt2 = 0;
for(int i=1; i<=n; i++){
char c;
cin>>c;
if(c == 'R') a[i] = 0, cnt0 ++;
else if(c == 'G') a[i] = 1, cnt1 ++;
else a[i] = 2, cnt2 ++;
}
for(int i=1; i<=n; i++){
q[a[i]].pb(i);
for(int d=0; d<3; d++){
cnt[i][d] = cnt[i-1][d] + (a[i] == d);
}
}
for(int i=0; i<=cnt0; i++){
for(int j=0; j<=cnt1; j++){
for(int k=0; k<=cnt2; k++){
for(int d=0; d<=2; d++){
dp[i][j][k][d] = INF;
}
}
}
}
for(int d=0; d<3; d++) dp[0][0][0][d] = 0;
for(int i=0; i<n; i++){
for(int x=0; x<=min(i, cnt0); x++){
for(int y=0; y<=min(i-x, cnt1); y++){
int z = i - x - y;
if(z > cnt2) continue;
if(x < cnt0)dp[x+1][y][z][0] = min(dp[x+1][y][z][0], min(dp[x][y][z][1], dp[x][y][z][2])+\
max(0, cnt[q[0][x]][1] - y) + max(0, cnt[q[0][x]][2] - z));
if(y < cnt1)dp[x][y+1][z][1] = min(dp[x][y+1][z][1], min(dp[x][y][z][0], dp[x][y][z][2])+\
max(0, cnt[q[1][y]][0] - x) + max(0, cnt[q[1][y]][2] - z));
if(z < cnt2)dp[x][y][z+1][2] = min(dp[x][y][z+1][2], min(dp[x][y][z][1], dp[x][y][z][0])+\
max(0, cnt[q[2][z]][1] - y) + max(0, cnt[q[2][z]][0] - x));
}
}
}
int ans = INF;
for(int i=0; i<3; i++){
ans = min(ans, dp[cnt0][cnt1][cnt2][i]);
}
if(ans < INF) cout<<ans;
else cout<<-1;
}
signed main(){
solve();
}
