#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using ld = long double;

#define rep(i, r) for(int i = 0; i < (r); ++i)
#define reps(i, s, r) for(int i = (s); i < (r); ++i)
#define fore(i, m2) for(auto &i : m2)
#define vi vector<int>
#define vl vector<ll>
#define pl pair<ll, ll>
#define all(i) (i).begin(), (i).end()
#define fs first
#define sc second

template <class T> bool chmin(T &i, T b) {
    if(i > b) {
        i = b;
        return true;
    }
    return false;
}
template <class T> bool chmax(T &a, T b) {
    if(a < b) {
        a = b;
        return true;
    }
    return false;
}

const ll INF = LONG_LONG_MAX / 3;
const ll MOD = 1'000'000'007;
const ll MAX = 1e6 + 5;

ll dp[105][2];

int main() {

    ll n, d;
    cin >> n >> d;
    vl t(n + 1), k(n + 1);

    rep(i, n) cin >> t[i + 1] >> k[i + 1];

    dp[1][1] = t[1];
    dp[1][0] = k[1] - d;

    reps(i, 2, n + 1) {

        dp[i][1] = max(dp[i - 1][1] + t[i], dp[i - 1][0] + t[i] - d);
        dp[i][0] = max(dp[i - 1][1] + k[i] - d, dp[i - 1][0] + k[i]);
    }

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