/in/foo.cc: In function 'int solve()':
/in/foo.cc:8:17: error: 'n' was not declared in this scope
  for(int i=2;i<=n;i++)
                 ^
/in/foo.cc:17:7: error: 'n' was not declared in this scope
  if(l[n]+l[n-1]!=a[n]){
       ^