#include <bits/stdc++.h>
#include <boost/multiprecision/cpp_int.hpp>
using namespace std;
namespace mp = boost::multiprecision;

int main(){
    int N,K;
    cin>>N>>K;
    int m=N,M=-N;
    for(int i=0;i<K;i++){
        int r,c;
        cin>>r>>c;
        M=max(M,r-c);
        m=min(m,r-c);
    }
    if(M>=0 && m<=0){
        cout<<-1<<endl;
    }else{
        cout<<(N+1-min(abs(M),abs(m)))<<endl;
    }
}