#define  _USE_MATH_DEFINES
#include <bits/stdc++.h>
#define rep(i, n) for (ll i = 0; i < (n); i++)
using namespace std;
using ll = long long;
using Graph = vector<vector<int>>;

int main() {
	int n;
	string s;
	cin >> n >> s;
	int ans = -1;
	if (s[0] != s[n - 1]) ans = 1;
	else {
		for (int i = 1; i < n - 1; i++) {
			if (s[0] != s[i] && s[i + 1] != s[n - 1]) ans = 2;
			break;
		}
	}
	cout << ans << endl;
}
#include<bits/stdc++.h>
using namespace std;
//#include <atcoder/modint>
//using namespace atcoder;
//using mint = modint998244353;
//using mint = modint1000000007;
#pragma region template
using ll = long long;
using PII = pair<int, int>;
using PLL = pair<ll, ll>;
template <class T> using V = vector<T>;
template <class T> using VV = V<V<T>>;
#define overload4(_1,_2,_3,_4,name,...) name
#define overload3(_1,_2,_3,name,...) name
#define rep1(n) for(ll i=0;i<n;++i)
#define rep2(i,n) for(ll i=0;i<n;++i)
#define rep3(i,a,b) for(ll i=a;i<b;++i)
#define rep4(i,a,b,c) for(ll i=a;i<b;i+=c)
#define rep(...) overload4(__VA_ARGS__,rep4,rep3,rep2,rep1)(__VA_ARGS__)
#define rrep1(n) for(ll i=n;i--;)
#define rrep2(i,n) for(ll i=n;i--;)
#define rrep3(i,a,b) for(ll i=b;i-->(a);)
#define rrep(...) overload3(__VA_ARGS__,rrep3,rrep2,rrep1)(__VA_ARGS__)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(x) ((int)(x).size())
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define debug(var)  do{std::cerr << #var << " : ";view(var);}while(0)
template<typename T> void view(T e){std::cerr << e << std::endl;}
template<typename T> void view(const std::vector<T>& v){for(const auto& e : v){ std::cerr << e << " "; } std::cerr << std::endl;}
template<typename T> void view(const std::vector<std::vector<T> >& vv){cerr << endl;int cnt = 0;for(const auto& v : vv){cerr << cnt << "th : "; view(v); cnt++;} cerr << endl;}
std::ostream &operator<<(std::ostream &dest, __int128_t value) {
  std::ostream::sentry s(dest);
  if (s) {
    __uint128_t tmp = value < 0 ? -value : value;
    char buffer[128];
    char *d = std::end(buffer);
    do {
      --d;
      *d = "0123456789"[tmp % 10];
      tmp /= 10;
    } while (tmp != 0);
    if (value < 0) {
      --d;
      *d = '-';
    }
    int len = std::end(buffer) - d;
    if (dest.rdbuf()->sputn(d, len) != len) {
      dest.setstate(std::ios_base::badbit);
    }
  }
  return dest;
}
template<class T, class K>bool chmax(T &a, const K b) { if (a<b) { a=b; return 1; } return 0; }
template<class T, class K>bool chmin(T &a, const K b) { if (b<a) { a=b; return 1; } return 0; }
const int inf = 1001001001;
const ll INF = 1001001001001001001ll;
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
ll rddiv(ll a, ll b) { return a/b-((a^b)<0&&a%b); } // -20 / 3 == -7
ll power(ll a, ll p){ll ret = 1; while(p){if(p & 1){ret = ret * a;} a = a * a; p >>= 1;} return ret;}
ll modpow(ll a, ll p, ll mod){ll ret = 1; while(p){if(p & 1){ret = ret * a % mod;} a = a * a % mod; p >>= 1;} return ret;}
ll rudiv(ll a, ll b) { return a/b+((a^b)>0&&a%b); } //  20 / 3 == 7
#pragma endregion
//const double pi = acos(-1);
//const ll mod = 998244353;
const ll mod = 1000000007;


int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    //cout << fixed << setprecision(20);
    int n; cin >> n;
    string s; cin >> s;
    if(s[0] == s.back()){
        char c = s[0];
        rep(i,n - 1){
            if(s[i] != c && s[i + 1] != c) cout << 2 << endl, exit(0);
        }
        cout << -1 << endl;
    }
    else cout << 1 << endl;
}