#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define llong long long

int main() {
    int n, d;
    cin >> n >> d;
    vector<int> t(n);
    vector<int> k(n);
    rep(i, n) cin >> t[i] >> k[i];

    vector<int> tokyo(n+1, 0);
    vector<int> kyoto(n+1, 0);
    tokyo[1] = t[0];
    kyoto[1] = k[0] - d;
    for(int i=1; i<n; i++) {
        tokyo[i+1] = max(tokyo[i] + t[i], kyoto[i] + t[i] - d);
        kyoto[i+1] = max(kyoto[i] + k[i], tokyo[i] + k[i] - d);
    }
    cout << max(tokyo[n], kyoto[n]) << "\n";
}