foo.cc: In function 'int get_max_len(int, int)':
foo.cc:30:6: warning: unused variable 'sum' [-Wunused-variable]
  int sum;
      ^~~
foo.cc: In function 'void readfile()':
foo.cc:22:6: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
 scanf("%d%d",&n,&m);
 ~~~~~^~~~~~~~~~~~~~
foo.cc:25:9: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
    scanf("%d",&num[i][j]);
    ~~~~~^~~~~~~~~~~~~~~~~

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