#include <bits/stdc++.h>

using namespace std;

#define rep(i, n) for(int i = 0; i < (n); ++i)
#define eee(a) (cerr << (#a) << ": " << (a) << endl);
#define ppp(a) (cout << (a) << endl); 
#define cop(a) for(auto x:(a)){cout << x << endl;}


using ll = long long;
using pint = pair<int, int>;
using pll = pair<ll, ll>;
using mint = map<int, int>;

int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

constexpr char sp = ' ';
constexpr char cmm = ',';
constexpr int MOD = 1e9+7;
constexpr int INF = 1e9;
constexpr ll LINF = 1e18;

ll mod(ll a, ll b){return (a%b+b)%b;}
ll gcd(ll a, ll b){return b?gcd(b,a%b):a;}
ll lcm(ll a, ll b){return a*b/gcd(a,b);}
inline void Yes(){cout << "Yes" << endl;}
inline void No(){cout << "No" << endl;}
inline void Judge(bool b){b?Yes():No();}
inline void YES(){cout << "YES" << endl;}
inline void NO(){cout << "NO" << endl;}
inline void JUDGE(bool b){b?YES():NO();}
ll powmod(ll b, ll e){ll r=1;while(e>0){if(e&1)r=(r%MOD)*(b%MOD)%MOD;b=(b%MOD)*(b%MOD)%MOD;e>>=1;}return r;}
double distance(ll x1, ll y1, ll x2, ll y2){return sqrt((x1-x2)*(x1-x2) + (y1-y2)*(y1-y2));}

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
template<typename T1, typename T2> ostream& operator<<(ostream& s, const pair<T1, T2>& p) {return s << "(" << p.first << ", " << p.second << ")";}
template<typename T> ostream& operator<<(ostream& s, const vector<T>& v) {
    int len = v.size();
    s << '[';
    for (int i = 0; i < len; ++i) {
        s << v[i]; if (i < len-1) s << ", ";
    }
    s << ']';
    return s;
}

struct aaa{
    aaa(){
        // comment this out when you use scanf or printf
        // cin.tie(0); ios::sync_with_stdio(0);
        cout<<fixed<<setprecision(15);
    };
}fastio;


int dp[105][2];
vector<int> t, k;
int main(){
    
    int n;
    int d;
    scanf("%d%d", &n, &d);
    t.resize(n);
    k.resize(n);
    rep(i, n) scanf("%d%d", &t[i], &k[i]);
   
    dp[0][0] = t.front(), dp[0][1] = k.front()-d;
    for(int i = 1; i < n; ++i){
        dp[i][0] = t[i] + max(dp[i-1][0], dp[i-1][1]-d);
        dp[i][1] = k[i] + max(dp[i-1][1], dp[i-1][0]-d);
    }
    ppp(max(dp[n-1][0], dp[n-1][1]));
    return 0;
}