#include <bits/stdc++.h>
using namespace std;
using ll = long long;

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

    ll N, D;
    cin >> N >> D;
    ll T[101] = {};
    ll K[101] = {};

    for (ll i = 0; i < N; i++) {
        cin >> T[i] >> K[i];
    }

    ll dp[101][2];
    dp[0][0] = T[0];
    dp[0][1] = K[0] - D;

    for (ll i = 1; i < N; i++) {
        dp[i][0] = max(dp[i - 1][0] + T[i], dp[i - 1][1] + T[i] - D);
        dp[i][1] = max(dp[i - 1][1] + K[i], dp[i - 1][0] + K[i] - D);
    }

    cout << (max(dp[N - 1][0], dp[N - 1][1])) << endl;

    return 0;
}