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// Karol Kosinski 2025
#include <bits/stdc++.h>
#define FOR(i,a,b) for(int i=(a),_b=(b);i<_b;++i)
#define FR_(i,a,b) for(int i=(a),_b=(b);i<=_b;++i)
#define FD_(i,b,a) for(int i=(b),_a=(a);i>=_a;--i)
#define ALL(c) (c).begin(),(c).end()
#define SIZE(c) int((c).size())
#define X first
#define Y second
#define endl '\n'
#define NAM(x) #x,'=',x,"  "
using namespace std;
using LL = long long;
using ULL = unsigned long long;
using PII = pair<int, int>;
using TIII = tuple<int, int, int>;
template<class...T> void _cout(T...a){(cout<<...<<a);}

#ifndef ENABLE_DEBUG
#define DEB(k,p,f,x...)
#else
#define DEB(k,p,f,x...) {if(k)_cout("------",setw(4),__LINE__," : ",__FUNCTION__,endl);if(p)f(x);}
#endif
#define DEBF(f,x...)    DEB(1,1,f,x)
#define DEBL            DEBF(void,0)
#define DEBC(p,x...)    DEB(0,p,_cout,x)
#define DEBUG(x...)     DEBC(1,x)

constexpr int NX = 500'003;

LL A[NX], H[NX];
int W[NX];

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    int n, c; cin >> n >> c;
    FOR(i,0,n) cin >> A[i] >> W[i];
    reverse( A, A + n );
    reverse( W, W + n );
    priority_queue< pair<LL, int> > Q;
    vector< pair<LL, int> > V;
    Q.emplace( 0, 0 );

    FOR(i,0,n)
    {
        auto [ cur, w ] = Q.top();
        DEBUG( "**** ", NAM(i), A[i], "  ", W[i], endl );
        DEBUG( "++ ", NAM(cur), NAM(w), endl );
        LL h = H[ W[i] ];
        if ( w == W[i] or h > cur - c )
        {
            h += A[i];
        }
        else
        {
            h = cur - c + A[i];
        }
        DEBUG( "++ ", NAM(h), endl );
        V.emplace_back( h, W[i] );
        if ( i == n - 1 or A[ i + 1 ] != A[i] )
        {
            while ( not V.empty() )
            {
                auto aux = V.back();
                Q.push(aux);
                H[ aux.Y ] = aux.X;
                V.pop_back();
            }
        }
    }
    LL res = 0;
    FOR(i,0,NX) res = max( res, H[i] );
    cout << res << endl;
    return 0;
}