#include <stdio.h>

int main () {
  int n = 0;
  int m = 0;
  int b[100000] = {};
  int c[100000] = {};
  
  int res = 0;
  
  int prev[100001] = {};
  
  int ap[100001] = {};
  int cnt = 0;
  int is_ok = 1;
  
  int ans = 0;
  
  res = scanf("%d", &n);
  res = scanf("%d", &m);
  for (int i = 0; i < m; i++) {
    res = scanf("%d", b+i);
    res = scanf("%d", c+i);
    if (c[i] < 0) {
      c[i] += (1+(-c[i])/b[i])*b[i];
    }
    if (prev[b[i]] > 0) {
      int idx = prev[b[i]]-1;
      if (c[i]%b[i] == c[idx]%b[idx]) {
        is_ok = 0;
      }
    } else {
      for (int j = c[i]%b[i]; j <= n; j += b[i]) {
        ap[j]++;
      }
      cnt++;
    }
    prev[b[i]] = i+1;
  }
  
  while (ans <= n) {
    if (ap[ans] >= cnt) {
      printf("%d\n", ans);
      return 0;
    }
    ans++;
  }
  
  printf("NaN\n");
  return 0;
}