#include <bits/stdc++.h>
using namespace std;
vector<int> sieve(int n) {
  vector<bool> prime(n + 1, true);
  prime[0] = false;
  prime[1] = false;
  int m = sqrt(n);
  vector<int> ans;
  for (int p = 2; p <= m; p++) {
    if (prime[p]) {
      for (int i = p * 2; i <= n; i += p) prime[i] = false;
    }
  }
  for (int i = 0; i < n; i++)
    if (prime[i]) ans.push_back(i);
  return ans;
}
int gcd(int a, int b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
int mod = 1e9 + 7;
int power(int a, long long b) {
  int res = 1;
  while (b) {
    if (b & 1) res = 1LL * res * a % mod;
    a = 1LL * a * a % mod;
    b >>= 1;
  }
  return res;
}
long long min(int a, long long b) {
  if (a > b) return b;
  return a;
}
long long max(int a, long long b) {
  if (a > b) return a;
  return b;
}
void solve() {
  long long n, m;
  cin >> n >> m;
  vector<pair<int, int>> p;
  for (int i = 0; i < m; i++) {
    int x, y;
    cin >> x >> y;
    p.push_back({x, y});
  }
  long long ans =
      max(p[0].first + p[0].second - 1, p[m - 1].second + (n - p[m - 1].first));
  for (int i = 1; i < m; i++) {
    if (abs(p[i].second - p[i - 1].second) > p[i].first - p[i - 1].first) {
      cout << "IMPOSSIBLE";
      return;
    }
    ans = max(ans, max(p[i].second, p[i - 1].second) +
                       (p[i].first - p[i - 1].first -
                        abs(p[i].second - p[i - 1].second)) /
                           2);
  }
  cout << ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int mul = 0;
  if (mul == 1) {
    int t;
    cin >> t;
    while (t--) {
      solve();
      cout << "\n";
    }
  } else {
    solve();
  }
}
