#include <iostream>
#include <vector>
#include <string>
#include "cstring"
#include <algorithm>
using namespace std;


int high(pair <int, int> a, pair <int, int> b)
{
    if (abs(b.second - a.second) > abs(b.first - a.first)) return -1;
    int l = -1, r = 100000000;
    while (l < r - 1)
    {
        int m = (l + r) / 2;
        pair <int, int> keep = make_pair(a.first + m, a.second + m);
        if (abs(b.second - keep.second) > abs(b.first - keep.first))
            r = m;
        else
            l = m;
    }
    return l;
}

int main()
{
    int n, m;
    cin >> n >> m;
    pair <int, int> last;
    int result = -1;
    for (int i = 0; i < m; i++)
    {
        pair <int, int> cur;
        cin >> cur.first >> cur.second;
        result = max(result, cur.second);
        if (i == m - 1)
            result = max(result, cur.second + n - cur.first);
        if (i != 0)
        {
            int q = high(last, cur);
            if (q == -1)
            {
                cout << "IMPOSSIBLE";
                return 0;
            }
            result = max(last.second + q, result);
        }
        else
            result = max(result, cur.second + cur.first - 1);
        last = cur;
    }
    cout << result;
    return 0;
}