Wrong Answer
/in/foo.cc: In function 'int main()':
/in/foo.cc:13:6: warning: operation on 'n' may be undefined [-Wsequence-point]
   a[n--]=(a[n]+a[n-1])/k;
     ~^~
/in/foo.cc:13:6: warning: operation on 'n' may be undefined [-Wsequence-point]
  /in/foo.cc: In function 'int main()':
/in/foo.cc:13:6: warning: operation on 'n' may be undefined [-Wsequence-point]
   a[n--]=(a[n]+a[n-1])/k;
     ~^~
/in/foo.cc:13:6: warning: operation on 'n' may be undefined [-Wsequence-point]