#include <bits/stdc++.h>
#define rep(i, a, n) for(int i = a; i < n; i++)
#define REP(i, n) rep(i, 0, n)
#define repb(i, a, b) for(int i = a; i >= b; i--)
#define all(a) a.begin(), a.end()
#define int long long
#define chmax(x, y) x = max(x, y)
#define chmin(x, y) x = min(x, y)
using namespace std;
typedef pair<int, int> P;
const int mod = 1000000007;
const int INF = 1e12;

signed main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n, d;
    cin >> n >> d;
    int t[110][2] = {};
    int now = 0, ans = 0;
    rep(i, 0, n){
        rep(j, 0, 2) cin >> t[i][j];
    }
    int dp[110][2] = {};
    dp[1][0] = t[0][0];
    dp[1][1] = t[0][1] - d;
    rep(i, 1, n){
        rep(j, 0, 2){
            int op = (j + 1) % 2;
            chmax(dp[i + 1][j], dp[i][j] + t[i][j]);
            chmax(dp[i + 1][op], dp[i][j] + t[i][op] - d);
        }
    }
    // rep(i, 0, n + 1){
    //     cout << dp[i][0] << " " << dp[i][1] << endl;
    // }
    cout << max(dp[n][0], dp[n][1]) << endl;
}