foo.c: In function 'main':
foo.c:14:12: warning: implicit declaration of function 'abs' [-Wimplicit-function-declaration]
   14 |         if(abs(num[i]-num[i+1])<=k)
      |            ^~~
foo.c:3:1: note: include '<stdlib.h>' or provide a declaration of 'abs'
    2 | #include<math.h>
  +++ |+#include <stdlib.h>
    3 | 

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