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#include<cstdio>
#include<iostream>
#include<algorithm>
#include<string>
#include<vector>
#include<cmath>
#include<queue>
#include<queue>
#include<stack>
#include<bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
#include <chrono>

using namespace std;
using namespace __gnu_pbds;

typedef vector<int> VI;
typedef vector<VI> VVI;
typedef long long LL;
typedef unsigned long long ULL;
typedef long double LD;
typedef pair<int, int> PII;
typedef pair<LL, LL> PLL;
typedef vector<LL> VLL;
typedef vector<LD> VLD;
typedef vector<VLL > VVLL;
typedef vector<VLD > VVLD;
typedef vector<PII > VPII;
typedef vector<PLL > VPLL;
typedef __int128 int128;
template<class TIn>
using indexed_set = tree<
        TIn, null_type, less<TIn>,
        rb_tree_tag, tree_order_statistics_node_update>;
template<class T> using min_heap = priority_queue<T, vector<T>, greater<T>>;

#define FOR(x, b, e) for(int x=b; x<=(e); ++x)
#define FORD(x, b, e) for(int x=b; x>=(e); --x)
#define REP(x, n) for(int x=0; x<(n); ++x)
#define VAR(v, n) __typeof(n) v = (n)
#define ALL(c) (c).begin(), (c).end()
#define SIZE(x) ((int)(x).size())
#define FOREACH(i, c) for(VAR(i, (c).begin()); i != (c).end(); ++i)
#define PB push_back
#define ST first
#define ND second
#define THIS (*this)
#define LSB(x) (x & -x)
#define SQR(x) ((x)*(x))

LL INF = 1e18;

class MaxTree {
    private:
    unordered_map<LL, LL> el;
    LL s = 1;

    public:
    MaxTree(LL size) {
        while (s < size) s <<= 1;
    }
    void set(LL p, LL c) {
        for(p += s, el[p] = c, p >>= 1; p; p >>= 1) el[p] = max(el[p << 1], el[(p << 1) + 1]);
    }
    LL get(LL l, LL r) {
        l += s; r += s;
        LL res = -INF;
        while(l < r) {
            if(l & 1) res = max(res, el[l++]);
            if(!(r & 1)) res = max(res, el[r--]);
            l >>= 1;
            r >>= 1;
        }
        if(l == r) res = max(res, el[l]);
        return res;
    }
};

int main() {
	ios_base::sync_with_stdio(0);
    cin.tie(NULL); cout.tie(NULL);
    int n, c; cin >> n >> c;
    VPII a(n);
    int maxPat = 0;
    REP(i, n) {
        cin >> a[i].ST >> a[i].ND;
        maxPat = max(maxPat, a[i].ND);
        a[i].ND--;
    }
    vector<pair<LL, int> > forPat(maxPat, {0, 1e9});
    MaxTree mt(a.back().ST + 1);
    FORD(i, n - 1, 0) {
        if (a[i].ST < forPat[a[i].ND].ND) {
            forPat[a[i].ND].ST += a[i].ST;
            forPat[a[i].ND].ND = a[i].ST;
        }
        auto best = mt.get(a[i].ST + 1, a.back().ST);
        if (best + a[i].ST - c > forPat[a[i].ND].ST) {
            forPat[a[i].ND].ST = best + a[i].ST - c;
            forPat[a[i].ND].ND = a[i].ST;
        }
        if (mt.get(a[i].ST, a[i].ST) < forPat[a[i].ND].ST) mt.set(a[i].ST, forPat[a[i].ND].ST);
    }
    LL ans = 0;
    REP(i, forPat.size()) ans = max(ans, forPat[i].ST);
    cout << ans << '\n';

    return 0;
}