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#include <bits/stdc++.h>

using namespace std;
#define pb push_back
#define st first
#define nd second
typedef long long ll;
typedef long double ld;
const ll I = 1000'000'000'000'000'000LL;
const int II = 2'000'000'000;
const ll M = 1000'000'007LL;
const int N = 1<<19;
int h[N], r[N];
ll dp[N];
ll drz[2 * N];

void Change(int v, ll x)
{
    v += N;
    while(v > 0)
    {
        drz[v] = max(drz[v], x);
        v /= 2;
    }
}

ll Query(int a, int b)
{
    a += N - 1; b += N + 1;
    ll ans = 0;
    while(a / 2 != b / 2)
    {
        if(a % 2 == 0) ans = max(ans, drz[a + 1]);
        if(b % 2 == 1) ans = max(ans, drz[b - 1]);
        a /= 2; b /= 2;
    }
    return ans;
}

void Solve()
{
    ll ans = 0LL;
    int n, c;
    cin >> n >> c;
    for(int i = 1; i <= n; ++i)
    {
        cin >> h[i] >> r[i];
        int j = i - 1;
        if(h[i] > h[i - 1])
            while(j > 0 && h[j] == h[i - 1])
            {
                Change(r[j], dp[j]);
                --j;
            }
        dp[i] = max(Query(1, r[i] - 1), Query(r[i] + 1, N - 10)) - (ll)c;
        dp[i] = max(dp[i], drz[r[i] + N]);
        dp[i] += (ll)h[i];
        ans = max(ans, dp[i]);
    }
    cout << ans << "\n";
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    //int t; cin >> t;
    //while(t--)
        Solve();

    return 0;
}