/*
 * =================================
 *
 * 	Task: magica
 * 	User: fanzeyi
 * 	Lang: C
 *
 * =================================
 */
#include <stdio.h>
#include <stdlib.h>

int main()
{
	FILE *fin=fopen("magica.in","r");
	FILE *fout=fopen("magica.out","w");
	int n,magic;
	int **things;
	fscanf(fin,"%d %d",&n,&magic);
	things=(int**)malloc(sizeof(int*)*n);
	int i;
	for(i=0;i<n;i++)
	{
		things[i]=(int*)malloc(sizeof(int)*2);
		things[i][1]=0;
		fscanf(fin,"%d %d",&things[i][0],&things[i][1]);
	}
	int count=0;
	for(i=0;i<n;i++)
	{
		if(things[i][1]==0)
		{
			count+=things[i][0];
			continue;
		}
		if(things[i][1]-things[i][0]>magic)
		{
			count-=magic;
			count+=things[i][1];
			continue;
		}
	}
	fprintf(fout,"%d",count);
	return 0;
}