#include <bits/stdc++.h>

 using namespace std;

 int n, m, x, y;

 bool check(int x, unordered_map<int, int>& m)

 {

     for (auto it : m)

     {

         int a = it.first, b = it.second;

         if ((a + x - ) / x != b) return false;

     }

     return true;

 }

 int main()

 {

     while (cin >> n >> m)

     {

         unordered_map<int, int> mp;

         for (int i = ; i < m; i++)

         {

             cin >> x >> y;

             mp[x] = y;

         }

         int ans = -;

         bool flg = true;

         for (int i = ; i <= ; i++)

         {

             if (check(i, mp))

             {

                 if (ans != - && (n + i - ) / i != ans)

                 {

                     flg = false; break;

                 }

                 else ans = (n + i - ) / i;

             }

         }

         if (flg && ans != -) cout << ans << endl;

         else cout << - << endl;

     }

     return ;

 }