#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()

int solve(){
    return 0;
}

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;
        bool check = false;
        for(auto p:ints)
            if(check = check || (p.first<=r && l<=p.second)) 
             { p.first=max(p.first,l);p.second=min(p.second,r);break;}

        if(!check) ints.push_back(make_pair(l,r));
    }    
    
    ans = N-ints.size();
    if(flg) prl(ans); else prl(-1);
//    if(flg) prl("Yes"); else prl("No");
}