foo.c: In function 'main':
foo.c:8:14: error: 'num' undeclared (first use in this function); did you mean 'sum'?
  scanf("%d",&num);
              ^~~
              sum
foo.c:8:14: note: each undeclared identifier is reported only once for each function it appears in
foo.c:5:9: warning: unused variable 'nun' [-Wunused-variable]
     int nun,i,k,sum;
         ^~~

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