#include<algorithm>
#include<complex>
#include<ctype.h>
#include<iomanip>
#include<iostream>
#include<fstream>
#include<map>
#include<math.h>
#include<numeric>
#include<queue>
#include<set>
#include<stack>
#include<stdio.h>
#include<string>
#include<string>
#include<vector>

using namespace std;
typedef long long ll;

#define FOR(i,a,b) for(ll i=(a);i<(b);++i)
#define ALL(v) (v).begin(), (v).end()
#define p(s) cout<<(s)<<endl
#define p2(s, t) cout << (s) << " " << (t) << endl
#define br() p("")
#define pn(s) cout << (#s) << " " << (s) << endl
#define p_yes() p("Yes")
#define p_no() p("No")

const ll mod = 1e9 + 7;
const ll inf = 1e18;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    // input
    ll N, D;
    cin >> N >> D;

    vector<ll> T(N);
    vector<ll> K(N);
    FOR(i, 0, N){
        cin >> T.at(i);
        cin >> K.at(i);
    }

    ll dp[2][N];
    dp[0][0] = T[0];
    dp[1][0] = K[0] - D;

    FOR(i, 1, N){
        dp[0][i] = max(dp[0][i-1] + T[i], dp[1][i-1] - D + T[i]);
        dp[1][i] = max(dp[1][i-1] + K[i], dp[0][i-1] - D + K[i]);
    }

    ll answer = max(dp[0][N-1], dp[1][N-1]);
    p(answer);
    
    return 0;
}