#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, e) for (int i = (int)(s); i < (int)(e); ++i)
#define all(a) (a).begin(),(a).end()

int main() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    
    int N, D;
    cin >> N >> D;
    vector dp(N + 1, vector<ll>(2, 0));
    dp[0][1] = -(1ll << 60);
    rep(i, 1, N + 1) {
        ll T, K;
        cin >> T >> K;
        dp[i][0] = max(dp[i - 1][0], dp[i - 1][1] - D) + T;
        dp[i][1] = max(dp[i - 1][1], dp[i - 1][0] - D) + K;
    }
    
    cout << max(dp[N][0], dp[N][1]) << '\n';
}