#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long double
#define REP(i,m,n) for(int i=(int)(m); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define RREP(i,m,n) for(int i=(int)(m); i>=(int)(n); i--)
#define rrep(i,n) RREP(i,(n)-1,0)
#define all(v) v.begin(), v.end()
#define endk '\n'
const int inf = 1e9+7;
const ll longinf = 1LL<<60;
const ll mod = 1e9+7;
const ll mod2 = 998244353;
const ld eps = 1e-10;
template<typename T1, typename T2> inline void chmin(T1 &a, T2 b){if(a>b) a=b;}
template<typename T1, typename T2> inline void chmax(T1 &a, T2 b){if(a<b) a=b;}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int n, k; cin >> n >> k;
  vector<vector<pair<int, int>>> V(3);
  rep(i, k) {
    int r, c; cin >> r >> c;
    if(r > c)       V[0].push_back({r, c});
    else if(r == c) V[1].push_back({r, c});
    else            V[2].push_back({r, c});
  }
  if(V[1].size()) cout << -1 << endk;
  else if(V[0].size() && V[2].size()) cout << -1 << endk;
  else if(V[0].size()) {
    int ans = 1;
    for(auto [r, c]: V[0]) chmax(ans, (n-r)+c+1);
    cout << ans << endk;
  }
  else {
    int ans = 1;
    for(auto [r, c]: V[2]) chmax(ans, r+(n-c)+1);
    cout << ans << endk;
  }
  return 0;
}