#include <bits/stdc++.h>
using namespace std;
#define MOD 1000000007
#define MOD2 998244353
#define rep(i,n) for (int i = 0; i < (int)(n); i++)
#define reps(i,s,n) for (int i = (int)(s); i < (int)(n); i++)
#define repit(i,C) for (auto i = (C).begin(); i != (C).end(); i++)
#define pr(a) cout << a
#define prl(a) cout << (a) << endl
#define prld(a) cout << setprecision(15)<< (a) << endl 
#define allrange(a) a.begin(),a.end()

pair<int,int> intersection(pair<int,int> &pa,pair<int,int> &pb){

    int l = max(pa.first,pb.first);
    int r = min(pa.second,pb.second);
    if(r<=l) return make_pair(0,0); 
    return make_pair(l,r);
}

int main(){
    std::cin.tie(0); // cinとcoutの同期を解除
    std::ios::sync_with_stdio(false);
//    int N;cin >> N;
    int N,M;cin >> N>>M;
//  int N,M,K;cin >> N>>M>>K;
    bool flg=true;
    int ans=0;

    vector<pair<int,int>> ints;
     
    rep(i,M){
        int l, r;
        cin >> l >> r;
        ints.push_back(make_pair(l,r+1));
    }

    sort(allrange(ints));
    
    const pair<int,int> nullp=make_pair(0,0);
    auto itr = ints.begin();
    pair<int,int> nint = *itr;
    itr++;
    int ct = 0;
    while(itr!= ints.end()){
        nint = intersection(nint,*itr);
        if(nint == nullp){
            nint = *itr;
            ct++;
        }
        itr++;
    }
    if(nint != nullp) ct++;
    ans = N-ct;
    if(flg) prl(ans); else prl(-1);
//    if(flg) prl("Yes"); else prl("No");
}