#include <bits/stdc++.h>
using namespace std;
typedef int ll;
ll visited[405][405][405][3];
ll dp[405][405][405][3];
ll tr,tg,ty,tt;
ll rr[405],gg[405],yy[405],r2[405],g2[405],y2[405];
ll dp2(ll r,ll g,ll y,ll last){
if (visited[r][g][y][last]==1) return dp[r][g][y][last];
visited[r][g][y][last] = 1;
if (r+g+y==1){
return 0;
}
ll ind,temp;
ll ar=r,ag=g,ay=y;
if (last==0){
ind = rr[tr-r+1];
temp = abs(r+g+y-1-ind+max(tg-g-g2[ind],0)+max(ty-y-y2[ind],0));
ar--;
}
else if (last==1){
ind = gg[tg-g+1];
temp = abs(r+g+y-1-ind+max(tr-r-r2[ind],0)+max(ty-y-y2[ind],0));
ag--;
}
else{
ind = yy[ty-y+1];
temp = abs(r+g+y-1-ind+max(tg-g-g2[ind],0)+max(tr-r-r2[ind],0));
ay--;
}
ll ans1=1000000,ans2=1000000,ans3=1000000;
if (ar>0 && last!=0) ans1 = dp2(ar,ag,ay,0)+temp;
if (ag>0 && last!=1) ans2 = dp2(ar,ag,ay,1)+temp;
if (ay>0 && last!=2) ans3 = dp2(ar,ag,ay,2)+temp;
ll ans = min(min(ans1,ans2),ans3);
dp[r][g][y][last] = ans;
// cout << r << " " << g << " " << y << " " << last << " " << ans << " " << ind << "\n";
return ans;
}
int main(){
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
ll t;
t=1;
while (t--){
ll n;
cin >> n;
string s;
cin >> s;
ll r1=1,g1=1,y1=1;
rr[0] = n,gg[0]=n,yy[0]=n;
for (int i=n-1;i>=0;i--){
if (s[i]=='R') {
rr[r1] = i;
r1++;
}
else if (s[i]=='G') {
gg[g1] = i;
g1++;
}
else {
yy[y1] = i;
y1++;
}
r2[i] = r1-1;
g2[i] = g1-1;
y2[i] = y1-1;
}
tr = r1-1;
ty = y1-1;
tg = g1-1;
tt = n;
ll ans1=1000000,ans2=1000000,ans3=1000000;
if (tr>0) ans1 = dp2(tr,tg,ty,0);
if (tg>0) ans2 = dp2(tr,tg,ty,1);
if (ty>0) ans3 = dp2(tr,tg,ty,2);
ll ans = min(min(ans1,ans2),ans3);
if (ans<500000) cout << ans;
else cout << -1;
// dp2(0,1,1,1);
}
}
