books.cpp: In function 'void solve(int, int, long long int, int)':
books.cpp:41:9: error: 'skim' was not declared in this scope
41 | if (skim(mid) < A) {
| ^~~~
books.cpp:50:8: error: 'impossible' was not declared in this scope
50 | else impossible();
| ^~~~~~~~~~
books.cpp:55:9: error: 'skim' was not declared in this scope
55 | if (skim(mid) > 2*A) {
| ^~~~
books.cpp:64:8: error: 'impossible' was not declared in this scope
64 | else impossible();
| ^~~~~~~~~~
books.cpp:70:5: error: 'answer' was not declared in this scope
70 | answer(ans);
| ^~~~~~
books.cpp:73:5: error: 'impossible' was not declared in this scope
73 | impossible();
| ^~~~~~~~~~
books.cpp: In function 'void iO()':
books.cpp:17:12: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
17 | freopen("output.txt","w",stdout);
| ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
books.cpp:18:12: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
18 | freopen("input.txt","r",stdin);
| ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~