// {{{ Templates
#include <bits/stdc++.h>

#define show(x) cerr << #x << " = " << x << endl

using namespace std;
using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;

template <typename T>
ostream& operator<<(ostream& os, const vector<T>& v)
{
    os << "sz:" << v.size() << "\n[";
    for (const auto& p : v) {
        os << p << ",";
    }
    os << "]\n";
    return os;
}

template <typename S, typename T>
ostream& operator<<(ostream& os, const pair<S, T>& p)
{
    os << "(" << p.first << "," << p.second
       << ")";
    return os;
}


constexpr ll MOD = (ll)1e9 + 7LL;

template <typename T>
constexpr T INF = numeric_limits<T>::max() / 100;

// }}}

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

    int N;
    cin >> N;
    ll D;
    cin >> D;
    vector<vector<ll>> T(N, vector<ll>(2, 0));
    for (int i = 0; i < N; i++) {
        cin >> T[i][0] >> T[i][1];
    }

    vector<vector<ll>> dp(N + 1, vector<ll>(2, -INF<ll>));  // day i; tokyo/kyoto
    dp[0][0] = 0;

    for (int i = 1; i <= N; i++) {
        for (int p = 0; p < 2; p++) {
            dp[i][p] = max(dp[i - 1][p] + T[i - 1][p], dp[i - 1][1 - p] + T[i - 1][p] - D);
        }
    }
    cout << max(dp[N][0], dp[N][1]) << endl;

    return 0;
}