foo.c: In function 'main':
foo.c:5:6: warning: unused variable 'ch' [-Wunused-variable]
 char ch='*';
      ^~
foo.c:4:13: warning: unused variable 's' [-Wunused-variable]
 int n,m,i,j,s;
             ^
foo.c:4:7: warning: unused variable 'm' [-Wunused-variable]
 int n,m,i,j,s;
       ^
foo.c:6:1: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
 scanf("%d",&n);
 ^~~~~~~~~~~~~~

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