#include <iostream>
#include <vector>
using namespace std;
int main()
{
	int d, c, p=0;	// day, cost, pay
	bool l = false;			// locate (tr=kyoto,fal=tongking)
	vector<int>t, k;	// tongking,kyoto
	cin >> d >> c;
	t.resize(d);
	k.resize(d);
	for (int i = 0; i < d; i++)	// entermoney
	{
		cin >> t[i] >> k[i];
	}

	for (int i = 0; i < d; i++)
	{
		if (l==false)
		{
			if (t[i] > k[i] - c)
			{
				p += t[i];
			}

			else if (t[i] < k[i] - c)
			{
				p += k[i] - c;
				l = true;
			}

			else
			{
				p += t[i];
				if (t[i + 1] < k[i + 1] - c) l = true;
			}
		}

		else
		{
			if (t[i] - c < k[i])
			{
				p += k[i];
			}

			else if (t[i] - c > k[i])
			{
				p += t[i] - c;
				l = false;
			}

			else
			{
				p += k[i];
				if (t[i + 1] - c > k[i + 1]) l = false;
			}
		}
	}

	cout << p << endl;
	return 0;
}