foo.c: In function 'main':
foo.c:8:5: warning: implicit declaration of function 'fun' [-Wimplicit-function-declaration]
  {n=fun(n);
     ^~~
foo.c: In function 'fun':
foo.c:14:5: warning: type of 'n' defaults to 'int' [-Wimplicit-int]
 int fun (n)
     ^~~
foo.c:19:6: error: invalid suffix "n" on integer constant
    n=3n+1;
      ^~
foo.c: In function 'main':
foo.c:6:5: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d",&n);
     ^~~~~~~~~~~~~~

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