func_code_string: #include <bits/stdc++.h>   using namespace std;   constexpr int inf = 1e9;   int main() {     int n, m;     cin >> n >> m;     vector<int> d(m), h(m);     for (int i = 0; i < m; ++i) {       cin >> d[i] >> h[i];     }     int ans = max(h[0] + d[0] - 1, h.back() + (n - d.back()));     for (int i = 0; i + 1 < m; ++i) {       if (abs(h[i] - h[i + 1]) > d[i + 1] - d[i]) {         ans = -1;         break;       }       ans = max(ans, (d[i + 1] - d[i] - abs(h[i] - h[i + 1])) / 2 +                          max(h[i], h[i + 1]));     }     if (ans == -1) {       cout <<  IMPOSSIBLE  << endl;     } else {       cout << ans << endl;     }   }   