#include<stdio.h>
#include<algorithm>
#include<map>
using namespace std;
typedef long long ll;
#define X first
#define Y second
typedef pair<int,int> Pi;
ll ans;
Pi stick[100010];
int n, L;
map <int, ll> M;
inline int get(Pi &a){return abs(a.X - a.Y) + L;}
int main(){
scanf("%d%d",&n,&L);
int i;
for(i=0;i<n;i++)scanf("%d%d",&stick[i].X,&stick[i].Y);
sort(stick, stick+n);
ll now = 0;
for(i=0;i<n;i++){
if(i == 0 || stick[i].X != stick[i-1].X){
ans = max(ans, now);
now = 0;
}
ll tmp = now + get(stick[i]), mp = M[stick[i].Y];
now = max(now, mp + get(stick[i]));
ans = max(ans, M[stick[i].Y] = max(mp, tmp));
}
printf("%lld",max(ans,now));
return 0;
}
