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

ll N, D;
ll T[101];
ll K[101];
ll ans = 0;

void dfs(ll wage, int type, int i) {
    if (i == N) {
        ans = max(ans, wage);
        return;
    }
    if (type == 1) {
        if (K[i] - T[i] >= D) {
            dfs(wage + K[i] - D, type * -1, i + 1);
        }
        dfs(wage + T[i], type, i + 1);
    } else {
        if (T[i] - K[i] >= D) {
            dfs(wage + T[i] - D, type * -1, i + 1);
        }
        dfs(wage + K[i], type, i + 1);
    }
}

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

    cin >> N >> D;

    for (ll i = 0; i < N; i++) {
        cin >> T[i] >> K[i];
    }
    dfs(0, 1, 0);

    cout << ans << endl;

    return 0;
}