//#include <bits/stdc++.h>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <iomanip>
#include <numeric>
#include <cmath>
#include <bit>
#include <cstdint>

#define rep(i,n) for(int i = 0; i < n; i++) //[0,n)
#define srep(i,a,b) for(int i = a; i < b; i++) //[a,b)
#define arep(i,a,b,d) for(int i = a; i < b; i += d)
#define all(A) (A).begin(),(A).end()
#define rall(A) (A).rbegin(),(A).rend()
#define pmt(A) next_permutation(all(A))
using namespace std;
using ll = long long;
using ull = unsigned long long;
using pq = priority_queue<ll,vector<ll>,greater<ll>>;
const ll inf = 1LL<<60;
const int iinf = (int)1e9+1;
const int mod9 = 998244353;
const int mod1 = (int)1e9+7;
struct Edge { int to; long long cost; int from; };
bool compe(const Edge &e,const Edge &e2){ return e.cost < e2.cost; }
using Graph = vector<vector<int>>;
using SGraph = vector<set<ll>>;
template <class T>
int siz(T& a){return (int)a.size();}
ll squa(ll a){ return a*a; }
double torad(double x){ return x*(acos(-1)/180.0); }
ll cross(ll ax,ll ay,ll bx,ll by){ return ax*by-ay*bx; }
ll mceil(ll a,ll b){ return (a+b-1)/b; }

int dp[110][3];
int main(){
    int N,D; cin >> N >> D;
    vector<int> T(N),K(N); rep(i,N) cin >> T[i] >> K[i];

    dp[0][0] = 0;
    dp[0][1] = -D;
    rep(i,N){
        rep(j,2){
            rep(k,2){
                if(j != k){
                    int add = (k == 0 ? T[i] : K[i]);
                    dp[i+1][k] = max(dp[i+1][k],dp[i][j]+add-D);
                }

                if(j == k){
                    int add = (j == 0 ? T[i] : K[i]);
                    dp[i+1][k] = max(dp[i+1][k],dp[i][j]+add);
                }
            }
        }
    }

    cout << max(dp[N][0],dp[N][1]);
}