#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1);
const double eps = 1e-9;
const int inf = 2000000000;
const long long infLL = 9000000000000000000;
inline bool checkBit(long long n, int i) { return n & (1LL << i); }
inline long long setBit(long long n, int i) {
  return n | (1LL << i);
  ;
}
inline long long resetBit(long long n, int i) { return n & (~(1LL << i)); }
int dx[] = {0, 0, +1, -1};
int dy[] = {+1, -1, 0, 0};
inline bool EQ(double a, double b) { return fabs(a - b) < 1e-9; }
inline bool isLeapYear(long long year) {
  return (year % 400 == 0) || (year % 4 == 0 && year % 100 != 0);
}
inline void normal(long long &a) {
  a %= 1000000007;
  (a < 0) && (a += 1000000007);
}
inline long long modMul(long long a, long long b) {
  a %= 1000000007, b %= 1000000007;
  normal(a), normal(b);
  return (a * b) % 1000000007;
}
inline long long modAdd(long long a, long long b) {
  a %= 1000000007, b %= 1000000007;
  normal(a), normal(b);
  return (a + b) % 1000000007;
}
inline long long modSub(long long a, long long b) {
  a %= 1000000007, b %= 1000000007;
  normal(a), normal(b);
  a -= b;
  normal(a);
  return a;
}
inline long long modPow(long long b, long long p) {
  long long r = 1;
  while (p) {
    if (p & 1) r = modMul(r, b);
    b = modMul(b, b);
    p >>= 1;
  }
  return r;
}
inline long long modInverse(long long a) { return modPow(a, 1000000007 - 2); }
inline long long modDiv(long long a, long long b) {
  return modMul(a, modInverse(b));
}
int a[105];
struct point {
  int k, f;
};
set<int> s;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  int n, m;
  cin >> n >> m;
  point x[m];
  for (int i = 0; i < m; ++i) {
    cin >> x[i].k >> x[i].f;
  }
  int ans = 0, pp;
  for (int i = 1; i <= 100; ++i) {
    memset(a, 0, sizeof(a));
    bool ok = 0;
    int key = i, story = 1;
    for (int j = 1; j <= 100; ++j) {
      a[j] = story;
      if (j % key == 0) {
        ++story;
      }
    }
    for (int j = 0; j < m; ++j) {
      if (a[x[j].k] != x[j].f) {
        ok = 1;
        break;
      }
    }
    if (!ok) {
      ++ans;
      pp = ceil((double)n / i);
      s.insert(ceil((double)n / i));
    }
  }
  if (s.size() > 1 || s.size() == 0)
    cout << -1;
  else
    cout << pp;
}
