foo.c:2:6: warning: return type of 'main' is not 'int' [-Wmain]
 void main()
      ^~~~
foo.c: In function 'main':
foo.c:6:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d%d",&m,&n);
  ^~~~~~~~~~~~~~~~~~~
foo.c:11:4: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
    scanf("%d",&a[i][j]);
    ^~~~~~~~~~~~~~~~~~~~
foo.c:58:11: warning: 'ni' may be used uninitialized in this function [-Wmaybe-uninitialized]
   if(b[ni][nj]==-1)
      ~~~~~^~~~
foo.c:58:11: warning: 'nj' may be used uninitialized in this function [-Wmaybe-uninitialized]

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