#include<stdio.h>
int main()
{
	int n, L , c;
	int count = 0;
	long long int x, y,  i, j,max = 0,min=0,t;
	int a[10001] = { 0 };
	scanf("%d %d", &n, &L);
	c = 0;
	while (n--)
	{
		
		scanf("%lld %lld", &x,&y);
		if (max < y)
		{
			max = y;
		}
		for (i = x; i < y; i += L)
		{
			for (j = 0; j < L; ++j)
			{
				t = i + j;

				if (a[t] == 0)
				{
					a[t] = 1;
				}
			}
		}	
	}
	for (i = 0;; i++)
		{
			if (a[i] == 1)
			{
				min = i;
				break;
			}
		}
		for (i = min; i < max; i += L)
		{
			count = 0;
			for (j = 0; j < L; ++j)
			{
				if (a[i + j] == 0)
					i = i + j + 1;
				if (a[i+j] == 1&&i+j<max)
				{
					count++;
				}
				
			}
			if (count ==L)
				c++;
		}
		printf("%d", c);
}