#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ar array
const int mxN = 410, M = 1e9+7;
int n, dp[4][210][210][210], sum[3][mxN], cnt[3];
string s;
vector<int> a[3];
int solvepos(char c, int pos){
if(c == 'R'){
return sum[0][pos+1];
}
if(c == 'G'){
return sum[1][pos+1];
}
return sum[2][pos+1];
}
int cost(int flag, int R, int G, int Y){
int pos;
if(flag == 0){
pos = a[flag][R-1];
}
if(flag == 1){
pos = a[flag][G-1];
}
if(flag == 2){
pos = a[flag][Y-1];
}
return max(solvepos('R', pos) - R, 0) + max(solvepos('G', pos) - G, 0) + max(solvepos('Y', pos) - Y, 0);
}
int solve(int last, int R, int G, int Y){
if(R+G+Y == 0){
return 0;
}
if(~dp[last][R][G][Y]) return dp[last][R][G][Y];
int flag = 1e9;
if(R && last != 0){
flag = min(flag, solve(0, R-1, G, Y) + cost(0, R, G, Y));
}
if(G && last != 1){
flag = min(flag, solve(1, R, G-1, Y) + cost(1, R, G, Y));
}
if(Y && last != 2){
flag = min(flag, solve(2, R, G, Y-1) + cost(2, R, G, Y));
}
return dp[last][R][G][Y] = flag;
}
int main(){
#ifdef _DEBUG
// freopen("input.txt", "r", stdin);
// freopen("output.txt", "w", stdout);
#endif
std::ios_base::sync_with_stdio(false); std::cin.tie(0); std::cout.tie(0);
cin >> n >> s;
for(int i = n-1; i>=0; --i){
if(s[i] == 'R'){
cnt[0]++;
a[0].push_back(i);
sum[0][i]++;
}else if(s[i] == 'G'){
cnt[1]++;
a[1].push_back(i);
sum[1][i]++;
}else{
cnt[2]++;
a[2].push_back(i);
sum[2][i]++;
}
for(int j = 0; j<3; ++j){
sum[j][i] += sum[j][i+1];
}
}
memset(dp, -1, sizeof(dp));
if(max(cnt[0], max(cnt[1], cnt[2])) > 202){
cout << "-1\n";
}else{
int ans = solve(3, cnt[0], cnt[1], cnt[2]);
if(ans >= 1e8){
cout << "-1\n";
}else{
cout << ans << "\n";
}
}
}
