Measure running time using chrono (C++11). Enable/disable printing the time by comment/uncomment the __EXE_CLOCK__ macro. This is also solution for Hashmat the Brave Warrior (UVa 10055), excluding the __EXE_CLOCK__ macro.

meaure-run-time.cpp

/*
sample run:
    ./measure-run-time < source-input.txt

output:
2
4
100

** elapsed time: 0.523ms **
*/

#include <iostream>
#include <cmath>

using namespace std;

//** CLOCK CODE START
#include <chrono>
auto start = chrono::high_resolution_clock::now();
#define __EXE_CLOCK__ \
	auto end = chrono::high_resolution_clock::now(); \
	auto elapsed = chrono::duration_cast<chrono::microseconds>(end - start); \
	cout << endl << "** elapsed time: " << float(elapsed.count())/1000.0 << "ms **" << endl;
//** CLOCK CODE END

using namespace std;

int main()
{
	long a, b;

	while (cin >> a >> b) {
		cout << abs(b - a) << endl;
	}

	__EXE_CLOCK__
}

source-input.txt

10 12
10 14
100 200