foo.c: In function 'main':
foo.c:4:19: error: 'o' undeclared (first use in this function)
     int i,a,n,num=o;
                   ^
foo.c:4:19: note: each undeclared identifier is reported only once for each function it appears in
foo.c:8:3: warning: this 'for' clause does not guard... [-Wmisleading-indentation]
   for(a=2;a<i;a++)
   ^~~
foo.c:11:4: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'for'
    if(a>=i)
    ^~
foo.c:5:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&n);
  ^~~~~~~~~~~~~~

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