books.cpp: In function 'int main()':
books.cpp:46:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
46 | freopen("input.txt", "r", stdin);
| ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
books.cpp:47:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
47 | freopen("output.txt", "w", stdout);
| ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/ld: /tmp/cc0HuvLo.o: in function `main':
grader.cpp:(.text.startup+0x0): multiple definition of `main'; /tmp/ccCK0Ktq.o:books.cpp:(.text.startup+0x0): first defined here
collect2: error: ld returned 1 exit status