Converting two's complement output to signed decimal

I apologize if this question has been answered already but I have not been able to find what I am looking for.

I am working in c++ with an SPI device. The SPI device outputs data in 16 bit words in 2's complement form. I am trying to convert this data into decimal for use with a filter.

I've attached some sample code that asks the user to input a number in twos complement and then outputs the signed decimal version.

#include <iostream>     
#include <stdlib.h>
#include <cstdint>
#include <cmath>
#include <bitset>
using std::cout;
using std::endl;
using std::cin;
using std::hex;
using std::dec;
using std::bitset;
int main () {
    uint16_t x2=0;
    cout<<"Please enter the number you would like to convert from 2's complement.  "<<endl;
    cin>>x2;
    int diff=0x0000-x2;
    cout<<"The number you have entered is: "<<dec<<diff<<endl;
return 0;
}

When I run this program and input something like 0x3B4A it always outputs 0. I'm not entirely sure what is going on and I'm very new to c++ so please excuse me if this is a stupid question. Also, please ignore anything extra in the header. This is partof a large project and I couldn't remember what parts of the header go with this specific section of code.

Thanks!

Edit: This is mostly for Ben. After reading your most recent comment I made the following changes but am still simply getting the decimal equivalent of the hexadecimal number I entered

#include <iostream>     
#include <stdlib.h>
#include <cstdint>
#include <cmath>
#include <bitset>
using std::cout;
using std::endl;
using std::cin;
using std::hex;
using std::dec;
using std::bitset;
int main () {
    int16_t x2=0;
    cout<<"Please enter the number you would like to convert from 2's complement.  "<<endl;
    cin>>hex>>x2;
    int flags= (x2>>14) & 3;
    int16_t value=(x2 << 2) >> 2;
    cout<<"The number you have entered is: "<<dec<<value<<endl;
return 0;
}