foo.c: In function 'is_prime':
foo.c:25:14: warning: implicit declaration of function 'sqrt' [-Wimplicit-function-declaration]
     int sq = sqrt(n);
              ^~~~
foo.c:25:14: warning: incompatible implicit declaration of built-in function 'sqrt'
foo.c:25:14: note: include '<math.h>' or provide a declaration of 'sqrt'
foo.c: In function 'main':
foo.c:7:5: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d", &n);
     ^~~~~~~~~~~~~~~

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