/in/foo.cc: In function 'int main()':
/in/foo.cc:7:8: error: 'a' was not declared in this scope
  {cin>>a[i];
        ^
/in/foo.cc:12:6: error: 'a' was not declared in this scope
  {if(a[i]!=s)a[i+1]=s-a[i],k++;
      ^