#include <bits/stdc++.h>
typedef long long ll;
using namespace std;
int dp[404][202][202][3], val[404][202][202][3];
vector<int> pos[3];
string s;
int f(char x){
if (x=='R') return 0;
else if (x=='G') return 1;
else if (x=='Y') return 2;
return -1;
}
bool CHK(int i, int j, int k, int z){
int l = i-j-k;
i--;
if (z==0) j--;
else if (z==1) k--;
else if (z==2) l--;
if (j<0 || k<0 || l<0) return 0;
if (j*2>i+1 || k*2>i+1 || l*2>i+1) return 0;
return 1;
}
int main(){
cin.tie(NULL);
ios_base::sync_with_stdio(false);
int n;
cin >> n >> s;
for (int i=0;i<n;i++){
pos[f(s[i])].push_back(i);
}
int r = pos[0].size(), g = pos[1].size(), y = pos[2].size();
if (r*2>n+1 || g*2>n+1 || y*2>n+1){
printf("-1\n");
return 0;
}
bool chk1=1;
for (int i=1;i<n;i++) if (s[i]==s[i-1]) chk1=0;
if (chk1){
printf("0\n");
return 0;
}
if (!pos[0].empty()) dp[1][1][0][0] = val[1][1][0][0] = pos[0][0];
if (!pos[1].empty()) dp[1][0][1][1] = val[1][0][1][1] = pos[1][0];
if (!pos[2].empty()) dp[1][0][0][2] = val[1][0][0][2] = pos[2][0];
for (int i=2;i<=n;i++){
for (int j=0;j<=min(i, r);j++){
for (int k=0;k<=min(i, g);k++){
int l = i-j-k;
if (l>y) continue;
if (l<0) break;
if (j){
if (!k && !l) val[i][j][k][0] = pos[0][i-1]-i+1;
else if (k) val[i][j][k][0] = val[i-1][j][k-1][0] - (pos[1][k-1]<pos[0][j-1]);
else val[i][j][k][0] = val[i-1][j][k][0] - (pos[2][l-1]<pos[0][j-1]);
}
if (k){
if (!j && !l) val[i][j][k][1] = pos[1][i-1]-i+1;
else if (j) val[i][j][k][1] = val[i-1][j-1][k][1] - (pos[0][j-1]<pos[1][k-1]);
else val[i][j][k][1] = val[i-1][j][k][1] - (pos[2][l-1]<pos[1][k-1]);
}
if (l){
if (!j && !k) val[i][j][k][2] = pos[2][i-1]-i+1;
else if (j) val[i][j][k][2] = val[i-1][j-1][k][2] - (pos[0][j-1]<pos[2][l-1]);
else val[i][j][k][2] = val[i-1][j][k-1][2] - (pos[1][k-1]<pos[2][l-1]);
}
}
}
/*printf(" %d\n", i);
for (int z=0;z<3;z++){
for (int j=0;j<=r;j++){
for (int k=0;k<=g;k++) printf("%d ", val[i][j][k][z]);
printf("\n");
}
printf("\n");
}
printf("\n");*/
}
///dp 계산
for (int i=2;i<=n;i++){
for (int j=0;j<=min(i, r);j++){
if (j*2>i+1) break;
for (int k=0;k<=min(i, g);k++){
if (k*2>i+1 || j+k>i) break;
int l = i-j-k;
if (l>y || l*2>i+1) continue;
if (l<0) break;
if (j){
int tmp1 = 1e9, tmp2 = 1e9;
if (CHK(i-1, j-1, k, 1)) tmp1 = dp[i-1][j-1][k][1];
if (CHK(i-1, j-1, k, 2)) tmp2 = dp[i-1][j-1][k][2];
if (min(tmp1, tmp2)!=1e9) dp[i][j][k][0] = min(tmp1, tmp2)+val[i][j][k][0];
}
if (k){
int tmp1 = 1e9, tmp2 = 1e9;
if (CHK(i-1, j, k-1, 0)) tmp1 = dp[i-1][j][k-1][0];
if (CHK(i-1, j, k-1, 2)) tmp2 = dp[i-1][j][k-1][2];
if (min(tmp1, tmp2)!=1e9) dp[i][j][k][1] = min(tmp1, tmp2)+val[i][j][k][1];
}
if (l){
int tmp1 = 1e9, tmp2 = 1e9;
if (CHK(i-1, j, k, 0)) tmp1 = dp[i-1][j][k][0];
if (CHK(i-1, j, k, 1)) tmp2 = dp[i-1][j][k][1];
if (min(tmp1, tmp2)!=1e9) dp[i][j][k][2] = min(tmp1, tmp2)+val[i][j][k][2];
}
}
}
/*printf(" %d\n", i);
for (int z=0;z<3;z++){
for (int j=0;j<=r;j++){
for (int k=0;k<=g;k++) printf("%d ", dp[i][j][k][z]);
printf("\n");
}
printf("\n");
}
printf("\n");*/
}
int ans=1e9;
for (int j=0;j<=r;j++){
for (int k=0;k<=g;k++){
for (int z=0;z<3;z++) if (dp[n][j][k][z]) ans = min(ans, dp[n][j][k][z]);
}
}
cout << ans;
/*while(true){
int q, w, e, r;
cin >> q >> w >>e >> r;
if (q==-1) break;
printf("%d %d %d\n", CHK(q, w, e, r), val[q][w][e][r], dp[q][w][e][r]);
}*/
return 0;
}
