#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
#define ordered_set tree<int, null_type, less<int>, rb_tree_tag,tree_order_statistics_node_update>
/*
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("avx,avx2,bmi2,fma,popcnt")
*/
#ifdef lisie_bimbi
#define debug(x) cout << #x << " : " << x << endl;
#else
#define debug(x) ;
#define endl '\n'
#endif
//#define int long long
#define inf 1000000000
#define double long double
typedef long long ll;
struct yeei{
int r;
int g;
int y;
};
signed main(){
#ifdef lisie_bimbi
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#else
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
#endif
int n;
cin >> n;
string s;
cin >> s;
int r = 0, g = 0, y = 0;
vector<int> vr, vg, vy;
for(int i = 0; i < n; i++){
if(s[i] == 'R'){
r++;
vr.push_back(i);
}else if(s[i] == 'G'){
g++;
vg.push_back(i);
}else{
y++;
vy.push_back(i);
}
}
vector<vector<vector<yeei>>> dp(r + 1, vector<vector<yeei>>(g + 1, vector<yeei>(y + 1, {inf, inf, inf})));
dp[0][0][0] = {0, 0, 0};
for(int r1 = 0; r1 <= r; r1++){
for(int g1 = 0; g1 <= g; g1++){
for(int y1 = 0; y1 <= y; y1++){
if((r1 == 0) && (g1 == 0) && (y1 == 0)){
continue;
}
if(r1 != 0){
int x = vr[r1 - 1];
int cnt = g1 + y1;
cnt -= min(g1, (int)(std::lower_bound(vg.begin(), vg.end(), x) - vg.begin()));
cnt -= min(y1, (int)(std::lower_bound(vy.begin(), vy.end(), x) - vy.begin()));
dp[r1][g1][y1].r = min(dp[r1][g1][y1].r, dp[r1 - 1][g1][y1].g + cnt);
dp[r1][g1][y1].r = min(dp[r1][g1][y1].r, dp[r1 - 1][g1][y1].y + cnt);
}
if(g1 != 0){
int x = vg[g1 - 1];
int cnt = r1 + y1;
cnt -= min(r1, (int)(std::lower_bound(vr.begin(), vr.end(), x) - vr.begin()));
cnt -= min(y1, (int)(std::lower_bound(vy.begin(), vy.end(), x) - vy.begin()));
dp[r1][g1][y1].g = min(dp[r1][g1][y1].g, dp[r1][g1 - 1][y1].r + cnt);
dp[r1][g1][y1].g = min(dp[r1][g1][y1].g, dp[r1][g1 - 1][y1].y + cnt);
}
if(y1 != 0){
int x = vy[y1 - 1];
int cnt = g1 + r1;
cnt -= min(g1, (int)(std::lower_bound(vg.begin(), vg.end(), x) - vg.begin()));
cnt -= min(r1, (int)(std::lower_bound(vr.begin(), vr.end(), x) - vr.begin()));
dp[r1][g1][y1].y = min(dp[r1][g1][y1].y, dp[r1][g1][y1 - 1].g + cnt);
dp[r1][g1][y1].y = min(dp[r1][g1][y1].y, dp[r1][g1][y1 - 1].r + cnt);
}
}
}
}
int x = min(dp[r][g][y].r, min(dp[r][g][y].g, dp[r][g][y].y));
if(x < inf){
cout << x;
}else{
cout << -1;
}
return 0;
}
