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#include <bits/stdc++.h>
#include <iomanip>
using namespace std;
#define fwd(i, a, n) for (int i = (a); i < (n); i++)
#define rep(i, n) fwd(i, 0, n)
#define all(X) X.begin(), X.end()
#define sz(X) int(size(X))
#define pb push_back
#define eb emplace_back
#define st first
#define nd second
using pii = pair<int, int>; using vi = vector<int>;
using ll = long long; using ld = long double;
#ifdef LOC
auto SS = signal(6, [](int) { *(int *)0 = 0; });
#define DTP(x, y) auto operator << (auto &o, auto a) -> decltype(y, o) { o << "("; x; return o << ")"; }
DTP(o << a.st << ", " << a.nd, a.nd);
DTP(for (auto i : a) o << i << ", ", all(a));
void dump(auto... x) { (( cerr << x << ", " ), ...) <<
'\n'; }
#define deb(x...) cerr << setw(4) << __LINE__ << ":[" #x "]: ", dump(x)
#else
#define deb(...) 0
#endif
 
const int MAXN = 5e5;
const ll INF = 1e18;

int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout << fixed << setprecision(10);
    ll n, c; cin >> n >> c;
    vector<ll> a(n);
    vector<int> w(n);
    rep(i, n)cin >> a[i] >> w[i];
    reverse(all(a));
    reverse(all(w));
    ll res = -INF;
    vector<ll> v(MAXN +1, -INF);
    ll best = -INF;
    vector<ll> dp(n, -INF);
    int j = 0;
    dp[0] = a[0];
    res = a[0];
    fwd(i, 1, n){
        while(a[j] > a[i]){
            best = max(best, dp[j]);
            v[w[j]] = max(v[w[j]], dp[j]);
            j++;
        }
        dp[i] = max(best - c, v[w[i]]) + a[i];
        dp[i] = max(dp[i], a[i]);
        res = max(res, dp[i]); 
    }
    deb(dp);
    cout << res << "\n";
    return 0;
}