#include<stdio.h>
#include<stdlib.h>
#include<math.h>
using namespace std;
int g,w,eg[30],cg[30],i,c;
int main()
{
    FILE*f=fopen("energii.in","r"); FILE*h=fopen("energii.out","w");
    fscanf(f,"%d%d",&g,&w);
    for(i=1;i<=g;i++)
    {
        fscanf(f,"%d%d",&eg[i],&cg[i]);
        c+=cg[i]; c-=eg[i];
    }
    c+=w;
    fprintf(h,"%d",c);
    fclose(f); fclose(h);
    return 0;
}