#include <algorithm>
#include <cmath>
#include <cstring>
#include <deque>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <string>
#include <unordered_map>
#include <vector>

using namespace std;
using ll = long long;

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

    ll N, D;
    ll T[110] = {};
    ll K[110] = {};
    cin >> N >> D;
    for (ll i = 0; i < N; i++) {
        cin >> T[i] >> K[i];
    }

    ll dp[110][2];
    for (ll i = 0; i < 110; i++) {
        dp[i][0] = 0;
        dp[i][1] = 0;
    }
    dp[0][1] = -1e9;

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

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

    return 0;
}