foo.cc: In function 'int main()':
foo.cc:54:6: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
 scanf( "%d%d" , &n , &m ) ;
 ~~~~~^~~~~~~~~~~~~~~~~~~~
foo.cc:56:18: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
 int s , t ; scanf( "%d%d" , &s , &t ) ;
             ~~~~~^~~~~~~~~~~~~~~~~~~~

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