#include <iostream>
#include <algorithm>

using namespace std;

int main(int argc, const char * argv[]) {
    int n;
    long long d;
    cin >> n >> d;
    long long tMax = 0, kMax = 0;
    for (int i = 0; i < n; ++i) {
        long long t, k;
        cin >> t >> k;
        long long tMaxNext = max(tMax + t, kMax + t - d);
        long long kMaxNext = (i == 0)
            ? tMax + k - d
            : max(tMax + k - d, kMax + k);
        tMax = tMaxNext;
        kMax = kMaxNext;
    }
    cout << max(tMax, kMax) << endl;
    return 0;
}