How do I derive and add new arguments to the base version of the constructor?

I'm trying to extend a base class with some data members and hence need some additional constructor arguments in addition to the constructor arguments that my base class needs. I want to forward the first constructor arguments to the base class. Here's what I tried:

#include <string>
#include <utility>

struct X
{
    X( int i_ ) : i(i_) {}
    int i;
};

struct Y : X
{
    template <typename ...Ts>        // note: candidate constructor not viable: 
    Y( Ts&&...args, std::string s_ ) // requires single argument 's_', but 2 arguments 
//  ^                                // were provided
    : X( std::forward<Ts>(args)... )
    , s( std::move(s_) )
    {}

    std::string s;
};

int main()
{
    Y y( 1, "" ); // error: no matching constructor for initialization of 'Y'
//    ^  ~~~~~
}

However, the compiler (clang 3.8, C++14 mode) spits the following error messages at me (the main messages are also in the above source code for reading convenience):

main.cpp:23:7: error: no matching constructor for initialization of 'Y'
    Y y( 1, "" );
      ^  ~~~~~
main.cpp:13:5: note: candidate constructor not viable: requires single argument 's_', but 2 arguments were provided
    Y( Ts&&...args, std::string s_ )
    ^
main.cpp:10:8: note: candidate constructor (the implicit move constructor) not viable: requires 1 argument, but 2 were provided
struct Y : X
       ^
main.cpp:10:8: note: candidate constructor (the implicit copy constructor) not viable: requires 1 argument, but 2 were provided
1 error generated.

Why is clang trying to tell me, that my templated constructor has only one arguments, even though the number of arguments is variadic? How can I solve this?