#include <bits/stdc++.h>
using namespace std;

#define REP(i,n) for(ll i=0;i<(ll)n;i++)
#define dump(x)  cerr << "Line " << __LINE__ << ": " <<  #x << " = " << (x) << "\n";
#define spa << " " <<
#define fi first
#define se second
#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()

using ld = long double;
using ll = long long;
using ull = unsigned long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pdd = pair<ld, ld>;

template<typename T> using V = vector<T>;
template<typename T> using P = pair<T, T>;
template<typename T> vector<T> make_vec(size_t n, T a) { return vector<T>(n, a); }
template<typename... Ts> auto make_vec(size_t n, Ts... ts) { return vector<decltype(make_vec(ts...))>(n, make_vec(ts...)); }
template<class S, class T> ostream& operator << (ostream& os, const pair<S, T> v){os << "(" << v.first << ", " << v.second << ")"; return os;}
template<typename T> ostream& operator<<(ostream &os, const vector<T> &v) { for (auto &e : v) os << e << ' '; return os; }
template<class T> ostream& operator<<(ostream& os, const vector<vector<T>> &v){ for(auto &e : v){os << e << "\n";} return os;}
struct fast_ios { fast_ios(){ cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;

template <class T> void UNIQUE(vector<T> &x) {sort(ALL(x));x.erase(unique(ALL(x)), x.end());}
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }
void fail() { cout << -1 << '\n'; exit(0); }
inline int popcount(const int x) { return __builtin_popcount(x); }
inline int popcount(const ll x) { return __builtin_popcountll(x); }
template<typename T> void debug(vector<vector<T>>&v,ll h,ll w){for(ll i=0;i<h;i++)
{cerr<<v[i][0];for(ll j=1;j<w;j++)cerr spa v[i][j];cerr<<"\n";}};
template<typename T> void debug(vector<T>&v,ll n){if(n!=0)cerr<<v[0];
for(ll i=1;i<n;i++)cerr spa v[i];
cerr<<"\n";};

const ll INF = (1ll<<62);
// const ld EPS   = 1e-10;
// const ld PI    = acos(-1.0);
const ll mod = (int)1e9 + 7;
//const ll mod = 998244353;

int main(){

    ll N, M;
    cin >> N >> M;
    V<ll> X(M), Y(M);
    REP(i, M) cin >> X[i] >> Y[i];

    set<ll> st;
    st.insert(N);

    V<ll> ids(M, 0);
    iota(ALL(ids), 0);
    sort(ALL(ids), [&](ll x, ll y){
        return X[x] < X[y];
    });

    REP(i, M){
        ll j = i;
        while(j < M and X[ids[i]] == X[ids[j]]){
            j++;
        }
        ll len = j - i;
        V<ll> check(len, 0);
        V<ll> vals;
        for(ll k=i;k<j;k++){
            vals.push_back(Y[ids[k]]);
        }
        REP(k, len){
            if(st.find(vals[k]-1) != st.end() or st.find(vals[k]+1) != st.end()){
                check[k] = 1;
            }
        }
        REP(k, len){
            if(check[k]){
                st.insert(vals[k]);
            }else{
                auto itr = st.find(vals[k]);
                if(itr != st.end()) st.erase(itr);
            }
        }

        i = j - 1;

        // for(auto x: st) cerr << x << " ";
        // cerr << endl;
    }

    cout << st.size() << endl;

    return 0;
}
#include<bits/stdc++.h>

using namespace std;

int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    #ifdef OJ
    freopen("input.txt","r",stdin);
    freopen("output.txt","w",stdout);
    freopen("error.txt","w",stderr);
    #endif
    int n;
    cin>>n;
    string a;
    cin>>a;
    if(a[0]!=a[a.size()-1])cout<<1;
    else 
    {
        char c1=a[0],c2=a[a.size()-1];
        for(int i=1;i<a.size();i++)
        {
            if(c1!=a[i] && c2!=a[i+1])
            {
                cout<<2;
                return 0;
            }
        }
        cout<<-1;
    }
    return 0;
}