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#include <bits/stdc++.h>
using namespace std;

typedef long long LL;
#define pb push_back

const LL N = 1e6 + 10;
constexpr LL base = 1 << 20;

LL tree[2*base];

void update(LL v, LL val)
{
    v += base;
    tree[v] = max(tree[v], val);
    v/=2;
    while(v)
    {
        tree[v] = max(tree[v*2], tree[v*2+1]);
        v/=2;
    }
}

LL query(LL a, LL b)
{
    a = a + base - 1;
    b = b + base + 1;

    LL res=0;
    while(a/2 != b/2)
    {
        if(a%2==0)
            res = max(res, tree[a+1]);
        if(b%2==1)
            res =max(res, tree[b-1]);

        a/=2;
        b/=2;
    }

    return res;
}

LL kolor[N];
LL tab[N];
LL dp[N];
vector<LL> prevv[N];
LL id[N];
vector<pair<LL,LL>> kolejka;

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);

    LL n, c;
    cin >> n >> c;
    LL res=0;
    LL wcz = 0, prevvvv = 0;
    for(LL i=1;i<=n;i++)
    {
        LL a, k;
        cin >> a >> k;
        kolor[i] = k;
        tab[i] = a;
    }

    for(LL i=1;i<=n;i++)
    {
        LL a = tab[i], k = kolor[i];
        LL ILE = max(query(0, k-1), query(k+1, 600000));
        dp[i] = max(dp[i], ILE + tab[i] - c);

        LL t = 0;
        LL L = 0, P = prevv[k].size()-1;
        while(L <= P)
        {
            LL mid = (L+P)/2;
            if(tab[prevv[k][mid]] < a)
            {
                L = mid + 1;
                t = prevv[k][mid];
            }
            else P = mid - 1;
        }

        dp[i] = max(dp[i], dp[t] + tab[i]);
        dp[i] = max(dp[i], tab[i]);
        res = max(res, dp[i]);
        prevv[k].pb(i);
        if(tab[i] != tab[i+1]) {
            for(auto v: kolejka)
                update(v.first, v.second);

            update(k, dp[i]);
            kolejka.clear();
        }
        else kolejka.pb({k, dp[i]});
    }

    cout << res;

}