#include <bits/stdc++.h>
using namespace std;
int main(){

	int n;
	string s;
	char a, b;
	cin >> n >> s;
	a = s[0];
	b = s[s.length()-1];
	bool ok = false;
	for(int i=1; i<s.length(); i++){
		if(s[i]!=a && s[i+1]!=b) ok= true;
	}
	if(a!=b) cout << "1";
	else if(ok) cout << "2";
	else cout << "-1";
	
    return 0;

}

#include <bits/stdc++.h>
#include <climits>

using namespace std;

template<class T>
using V = vector<T>;
template<class T>
using VV = V<V<T>>;

using ld = long double;
#define ll long long
using ull = unsigned ll;
using PLL = pair<ll, ll>;
using VLL = V<ll>;
using VB = V<bool>;
using VVB = VV<bool>;
using VVLL = VV<ll>;
using Gr = VVLL;
using MLL = map<ll, ll>;
#define UMLL unordered_map<ll, ll, custom_hash>

#define fast ios_base::sync_with_stdio(0); cin.tie(nullptr); cout.tie(nullptr); cerr.tie(nullptr);

#define R &
#define CR const R

#define FORI(i, a, b) for(ll i = a, max##i = b; i < max##i; ++i)
#define FOR(i, n) FORI(i, 0, n)
#define RFORI(i, a, b) for(ll i = a, min##i = b; i >= min##i; --i)
#define RFOR(i, n) RFORI(i, n, 0)
#define FORA(i, a) for(auto i : a)
#define FORAR(i, a) for(auto R i : a)
#define FORACR(i, a) for(auto CR i : a)
#define ALL(obj) begin(obj), end(obj)
#define Count(q) while(q--)
#define OK cerr << "OK\n";

#define mp make_pair
#define pb push_back

//#define DEBUG

template<class T>
T sqr(T x)
{
    return x * x;
}

void YES(bool g, ostream R os, bool upper = true)
{
    if(g)
        if(upper)
            os << "YES";
        else
            os << "Yes";
    else
        if(upper)
            os << "NO";
        else
            os << "No";

    os << "\n";
}

template<class T>
void show(T CR t, ostream R os = cerr)
{
    FORACR(i, t)
        os << i << " ";
    os << "\n";
}

template<class T>
void show2d(T CR t, ostream R os = cerr)
{
    FORACR(i, t)
        show(i, os);
    os << "\n";
}

constexpr ll MOD = 1e9 + 7; //998244353;
constexpr ll len = 'z' - 'a' + 1;

constexpr ll INF = 1e15, MINF = -INF;

constexpr ld PI = atanl(1.0L) * 4;
constexpr ld eps = 1e-9, EPS = 1e-9;

struct custom_hash
{
    static uint64_t splitmix64(uint64_t x)
    {
        x += 0x9e3779b97f4a7c15;
        x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
        x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
        return x ^ (x >> 31);
    }

    size_t operator() (uint64_t x) const
    {
        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
        return splitmix64(x + FIXED_RANDOM);
    }
};

//arr
void init() {}

void solve(istream R is, ostream R os)
{
    ll n;
    is >> n;

    string s;
    is >> s;

    char t = s[0];
    bool g = 1;

    FORA(i, s)
        g &= i == t;

    if(g)
    {
        os << "-1\n";
        return;
    }

    if(s.front() != s.back())
    {
        os << "1\n";
        return;
    }

    g = 0;
    FOR(i, n - 1)
        if(t != s[i] && t != s[i + 1])
        {
            g = 1;
        }

    if(g)
        os << "2\n";
    else
        os << "-1\n";
}

void tester(istream R is, ostream R os)
{
    auto seed = time(nullptr);
    //cerr << "seed: " << seed << "\n";
    srand(seed);
    fast
    init();
    ll q = 1;
    //is >> q;

    os << setprecision(999);
    Count(q)
        solve(is, os);
}

int main()
{
    //ifstream in("input.txt");
    //ofstream out("output.txt");

    tester(cin, cout);
}
