# include <bits/stdc++.h>
# define N 100010
char s[N];
int main() {
	int n,i,las=0;
	scanf("%d%s",&n,s+1);
	if(s[1]!=s[n]) puts("1");
	else {
		for(i=2;i<n-1;i++) {
			if(s[i]!=s[1] && s[i+1]!=s[1]) break;
		}
		if(i<n-1) puts("2");
		else puts("-1");
	}
	return 0;
}
#include <bits/stdc++.h>
#define FASTIO
using namespace std;

using ll = long long;
using Vi = std::vector<int>;
using Vl = std::vector<ll>;
using Pii = std::pair<int, int>;
using Pll = std::pair<ll, ll>;

constexpr int I_INF = std::numeric_limits<int>::max();
constexpr ll L_INF = std::numeric_limits<ll>::max();

template <typename T1, typename T2>
inline bool chmin(T1& a, const T2& b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template <typename T1, typename T2>
inline bool chmax(T1& a, const T2& b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

template <std::ostream& os = std::cout>
class Prints {
private:
    class __Prints {
    public:
        __Prints(const char* sep, const char* term) : sep(sep), term(term) {}
        template <class... Args>
#if __cplusplus >= 201703L
        auto operator()(const Args&... args) const -> decltype((os << ... << std::declval<Args>()), void()) {
#else
        void operator()(const Args&... args) const {
#endif
            print(args...);
        }
        template <typename T>
#if __cplusplus >= 201703L
        auto pvec(const T& vec, size_t sz) const -> decltype(os << std::declval<decltype(std::declval<T>()[0])>(), void()) {
#else
        void pvec(const T& vec, size_t sz) const {
#endif
            for (size_t i = 0; i < sz; i++)
                os << vec[i] << (i == sz - 1 ? term : sep);
        }
        template <typename T>
#if __cplusplus >= 201703L
        auto pmat(const T& mat, size_t h, size_t w) const -> decltype(os << std::declval<decltype(std::declval<T>()[0][0])>(), void()) {
#else
        void pmat(const T& mat, size_t h, size_t w) const {
#endif
            for (size_t i = 0; i < h; i++)
                for (size_t j = 0; j < w; j++)
                    os << mat[i][j] << (j == w - 1 ? term : sep);
        }

    private:
        const char *sep, *term;
        void print() const { os << term; }
        void print_rest() const { os << term; }
        template <class T, class... Tail>
        void print(const T& head, const Tail&... tail) const { os << head, print_rest(tail...); }
        template <class T, class... Tail>
        void print_rest(const T& head, const Tail&... tail) const { os << sep << head, print_rest(tail...); }
    };

public:
    Prints() {}
    __Prints operator()(const char* sep = " ", const char* term = "\n") const { return __Prints(sep, term); }
};

Prints<> prints;
Prints<std::cerr> prints_err;

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

void solve() {
    ll N;
    cin >> N;
    string S;
    cin >> S;
    char fc = S.front();
    char bc = S.back();
    if (fc != bc) {
        prints()(1);
        return;
    }
    for (ll i = 1; i < N - 2; i++) {
        if (S[i] != fc && S[i + 1] != fc) {
            prints()(2);
            return;
        }
    }
    prints()(-1);
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

int main() {
#ifdef FASTIO
    std::cin.tie(nullptr), std::cout.tie(nullptr);
    std::ios::sync_with_stdio(false);
#endif
#ifdef FILEINPUT
    std::ifstream ifs("./in_out/input.txt");
    std::cin.rdbuf(ifs.rdbuf());
#endif
#ifdef FILEOUTPUT
    std::ofstream ofs("./in_out/output.txt");
    std::cout.rdbuf(ofs.rdbuf());
#endif
    std::cout << std::setprecision(18) << std::fixed;
    solve();
    std::cout << std::flush;
    return 0;
}