func_code_string: #include <bits/stdc++.h>   using namespace std;   void no() {     cout <<  IMPOSSIBLE  << endl;     exit(0);   }   int main() {     cin.sync_with_stdio(false);     int n, m;     cin >> n >> m;     int answer = 0;     int last_day = 0, last_h = 0;     for (int i = 0; i < m; i++) {       int cur, h;       cin >> cur >> h;       if (i == 0) {         answer = h + cur - 1;       } else {         answer = max(answer, h);         int dif = abs(h - last_h);         int days = cur - last_day;         if (dif > days) no();         if (h >= last_h) {           answer = max(answer, h + (days - dif) / 2);         } else {           answer = max(answer, last_h + (days - dif) / 2);         }       }       last_day = cur;       last_h = h;     }     answer = max(answer, last_h + n - last_day);     cout << answer << endl;     return 0;   }   