func_code_string: #include <bits/stdc++.h>   using namespace std;   const long long N = 1e5 + 5;   long long n, m, ans = 0;   long long d[N], h[N];   int32_t main() {     ios::sync_with_stdio(0);     cin.tie(0);     cout.tie(0);     ;     cin >> n >> m;     for (long long i = 1; i <= m; i++) cin >> d[i] >> h[i];     for (long long i = 2; i <= m; i++) {       long long maxdiff = abs(d[i] - d[i - 1]);       if (abs(h[i] - h[i - 1]) > maxdiff) {         cout <<  IMPOSSIBLE ;         return 0;       }     }     ans = max(h[1] + abs(d[1] - 1), h[m] + abs(n - d[m]));     for (long long i = 1; i <= m - 1; i++) {       long long extradays = d[i + 1] - d[i] - abs(h[i + 1] - h[i]);       long long maxh = max(h[i], h[i + 1]);       ans = max(ans, maxh + extradays / 2);     }     cout << ans;     return 0;   }   