#include <cstdio>
#include <cstring>
#include <vector>
#include <map>
#include <algorithm>
using namespace std;
#define MAXN 100001
typedef long long ll;
struct Stick {
int t, d;
};
int N, L;
Stick A[MAXN];
map<int,vector<int>> B, C; // t, d
ll cache[MAXN];
ll solve(int p) {
ll &ret = cache[p];
if (ret != -1) return ret;
ret = 0;
vector<int> vB = B[A[p].t];
vector<int> vC = C[A[p].d];
for (int i=0; i<vB.size(); ++i)
if (p!=vB[i] && A[vB[i]].d>A[p].d)
ret = max(ret, solve(vB[i]));
for (int i=0; i<vC.size(); ++i)
if (p!=vC[i] && A[vC[i]].t>A[p].t)
ret = max(ret, solve(vC[i]));
return ret=ret+L+abs(A[p].t-A[p].d);
}
int main() {
memset(cache,-1,sizeof cache);
scanf("%d%d", &N, &L);
for (int i=1; i<=N; ++i) {
scanf("%d%d", &A[i].t, &A[i].d);
B[A[i].t].push_back(i);
C[A[i].d].push_back(i);
}
ll sol = 0;
for (int i=1; i<=N; ++i)
sol = max(sol, solve(i));
printf("%lld", sol);
}
