/in/foo.c: In function 'main':
/in/foo.c:4:20: warning: variable 'sum' set but not used [-Wunused-but-set-variable]
  int f[21],i,k1,k2,sum;
                    ^~~
/in/foo.c:13:5: warning: 'k2' is used uninitialized in this function [-Wuninitialized]
  sum=k2-k1;
  ~~~^~~~~~
/in/foo.c:13:5: warning: 'k1' is used uninitialized in this function [-Wuninitialized]