/in/foo.cc: In function 'int main()':
/in/foo.cc:14:46: error: invalid types 'long long int[long long int]' for array subscript
  for(i=1;i<=10;i++)scanf("%lld %lld",&a[i*2-1],&a[i*2]);
                                              ^
/in/foo.cc:14:54: error: invalid types 'long long int[long long int]' for array subscript
  for(i=1;i<=10;i++)scanf("%lld %lld",&a[i*2-1],&a[i*2]);
                                                      ^
/in/foo.cc:15:35: warning: format '%lld' expects a matching 'long long int' argument [-Wformat=]
  printf("%lld\n%lld%lld%lld",n,a,b);
                                   ^