#include <bits/stdc++.h>
//#include "includeall.h"
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
#define endl '\n'
#define f first
#define s second
#define pb push_back
#define mp make_pair
#define lb lower_bound
#define ub upper_bound
#define input(x) scanf("%lld", &x);
#define input2(x, y) scanf("%lld%lld", &x, &y);
#define input3(x, y, z) scanf("%lld%lld%lld", &x, &y, &z);
#define input4(x, y, z, a) scanf("%lld%lld%lld%lld", &x, &y, &z, &a);
#define print(x, y) printf("%lld%c", x, y);
#define show(x) cerr << #x << " is " << x << endl;
#define show2(x,y) cerr << #x << " is " << x << " " << #y << " is " << y << endl;
#define show3(x,y,z) cerr << #x << " is " << x << " " << #y << " is " << y << " " << #z << " is " << z << endl;
#define all(x) x.begin(), x.end()
#define discretize(x) sort(x.begin(), x.end()); x.erase(unique(x.begin(), x.end()), x.end());
#define FOR(i, x, n) for (ll i =x; i<=n; ++i)
#define RFOR(i, x, n) for (ll i =x; i>=n; --i)
#pragma GCC optimize("O3","unroll-loops")
using namespace std;
mt19937_64 rnd(chrono::steady_clock::now().time_since_epoch().count());
//using namespace __gnu_pbds;
//#define ordered_set tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>
//#define ordered_multiset tree<int, null_type, less_equal<int>, rb_tree_tag, tree_order_statistics_node_update>
typedef long long ll;
typedef long double ld;
typedef pair<ld, ll> pd;
typedef pair<string, ll> psl;
typedef pair<ll, ll> pi;
typedef pair<ll, pi> pii;
typedef pair<pi, pi> piii;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
ll dp[405][405][405][3];
int main()
{
ll n, r = 0, g = 0, y = 0;
string ss;
cin >> n >> ss;
vector<ll> rr, gg, yy;
for (ll i=0; i<n; ++i)
{
if (ss[i]=='R') { r++; rr.pb(i);}
else if (ss[i]=='G') { g++; gg.pb(i);}
else { y++; yy.pb(i);}
}
for (ll i=0; i<=r; ++i) for (ll j=0; j<=g; ++j) for (ll k=0; k<=y; ++k) for (ll x=0; x<3; ++x) dp[i][j][k][x] = ll(1e15);
dp[0][0][0][0] = dp[0][0][0][1] = dp[0][0][0][2] = 0;
for (ll i=0; i<=r; ++i) for (ll j=0; j<=g; ++j) for (ll k=0; k<=y; ++k)
{
if (i) // 0
{
ll idx = rr[i-1], c = min(dp[i-1][j][k][1], dp[i-1][j][k][2]) + max(0LL, lb(all(yy), idx)-yy.begin() - k) + max(0LL, lb(all(gg), idx)-gg.begin() - j);
dp[i][j][k][0] = min(dp[i][j][k][0], c);
}
if (j) // 1
{
ll idx = gg[j-1], c = min(dp[i][j-1][k][0],dp[i][j-1][k][2]) + max(0LL, lb(all(yy), idx)-yy.begin() - k) + max(0LL, lb(all(rr), idx)-rr.begin() - i);
dp[i][j][k][1] = min(dp[i][j][k][1], c);
}
if (k) // 2
{
ll idx = yy[k-1], c = min(dp[i][j][k-1][0], dp[i][j][k-1][1]) + max(0LL, lb(all(rr), idx)-rr.begin() - i) + max(0LL, lb(all(gg), idx)-gg.begin() - j);
dp[i][j][k][2] = min(dp[i][j][k][2], c);
}
//show3(i, j, k);
//show(dp[i][j][k][0]);
//show(dp[i][j][k][1]);
//show(dp[i][j][k][2]);
}
ll ans = *min_element(dp[r][g][y], dp[r][g][y]+3);
cout << ((ans>=ll(1e15))?-1:ans) << endl;
return 0;
}
