foo.cpp: In function 'int find(int, int, int)':
foo.cpp:104:6: warning: unused variable 'mid' [-Wunused-variable]
  int mid=(l+r)/2;
      ^~~
foo.cpp: In function 'void read(int&)':
foo.cpp:15:16: warning: 'ch' is used uninitialized in this function [-Wuninitialized]
  while ((ch<'0'||ch>'9')&&ch!='-') ch=getchar();
         ~~~~~~~^~~~~~~~~