#include<iostream>
#include<cstdio>
using namespace std;
#define fo(i,a,b) for(int i=a;i<=b;++i)
#define fod(i,a,b) for(int i=a;i>=b;--i)
#define LL long long
LL read(){
	LL u=0;
	char c=getchar();
	while(c>'9'||c<'0')c=getchar();
	while(c>='0'&&c<='9')u=u*10+c-48,c=getchar();
	return u;
}
int n;
LL r2,s[200011],w[200011];
int solve_l(LL l){
	LL R=0;
	fo(i,1,n){
		R=min(R+w[i],min(r2,R+w[i]-l+s[i]));
		if(R<0)return 0;
	}
	return 1;
}
int solve_r(LL r){
	LL L=0;
	fo(i,1,n){
		L=L+w[i]-r+s[i];
		if(L<0)L=0;
		if(L>r2||(i==n&&L>0))return 0;
	}
	return 1;
}
int main(){
	n=read();
	r2=read();
	fo(i,1,n){
		s[i]=read();
		w[i]=read();
	}
	LL ans=0;
	LL l=0,r=2000000001,mid;
	while(l<r){
		mid=(l+r+1)/2;
		if(solve_l(mid))l=mid;
		else r=mid-1;
	}
	ans-=l;
	l=0,r=2000000001,mid;
	while(l<r){
		mid=(l+r)/2;
		if(solve_r(mid))r=mid;
		else l=mid+1;
	}
	printf("%lld",ans+r);
	return 0;
}