#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template <typename T>
bool chmax(T& a, const T& b)
{
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
template <typename T>
bool chmin(T& a, const T& b)
{
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
constexpr ll inf = (1LL << 62);
long dp[110][2];
int main()
{
    ll n, d;
    cin >> n >> d;
    vector<ll> t(n), k(n);
    for (ll i = 0; i < n; ++i) {
        cin >> t[i] >> k[i];
    }
    dp[0][0] = 0;
    dp[0][1] = -d;
    for (ll i = 0; i < n; ++i) {
        dp[i + 1][0] = max(dp[i][1] - d + t[i], dp[i][0] + t[i]);
        dp[i + 1][1] = max(dp[i][0] - d + k[i], dp[i][1] + k[i]);
    }
    cout << max(dp[n][0], dp[n][1]) << endl;
}