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

int main() {
    long long N, D;
    cin >> N >> D;
    long long DP[N + 1][2] = {};
    for (size_t i = 1; i <= N; i++) {
        long long T, K;
        cin >> T >> K;
        if (i == 1) {
            DP[i][0] = T;
            DP[i][1] = K - D;
            continue;
        }
        DP[i][0] = max(DP[i - 1][0] + T, DP[i - 1][1] + T - D);
        DP[i][1] = max(DP[i - 1][0] + K - D, DP[i - 1][1] + K);
    }
    cout << max(DP[N][0], DP[N][1]) << endl;
}