foo.c: In function 'main':
foo.c:15:2: warning: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
   15 |  scanf("%d",&n);
      |  ^~~~~
foo.c:15:2: warning: incompatible implicit declaration of built-in function 'scanf'
foo.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | int isPrime(int x)
foo.c:19:2: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   19 |  printf("%d\n",count);
      |  ^~~~~~
foo.c:19:2: warning: incompatible implicit declaration of built-in function 'printf'
foo.c:19:2: note: include '<stdio.h>' or provide a declaration of 'printf'

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