foo.c: In function 'main':
foo.c:9:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d%d", &m, &n); //m行, n列
  ^~~~~~~~~~~~~~~~~~~~~
foo.c:15:4: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
    scanf("%d", &a[i][j]); 
    ^~~~~~~~~~~~~~~~~~~~~

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