#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, j) for(int i=0; i < (int)(j); i++)
#define all(v) v.begin(),v.end()
template<class T> bool set_max(T &a, const T &b) { return a < b  ? a = b, true : false; }
const int INF = 1 << 30;
const ll INFL = 1LL << 60;


class Solver {
  public:
    bool solve() {
        int N, D; cin >> N >> D;
        vector<vector<ll>> dp(N + 1, vector<ll>(2, -INFL));
        dp[0][0] = 0;
        rep(n, N) {
            ll T, K; cin >> T >> K;
            set_max(dp[n + 1][0], dp[n][0] + T);
            set_max(dp[n + 1][0], dp[n][1] + T - D);
            set_max(dp[n + 1][1], dp[n][0] + K - D);
            set_max(dp[n + 1][1], dp[n][1] + K);
        }
        cout << *max_element(all(dp[N])) << endl;
        return 0;
    }
};

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    Solver s;
    s.solve();
    return 0;
}