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#ifndef LOCAL
#pragma GCC optimize("O3")
#endif
#include <bits/stdc++.h>
#define FOR(i,p,k) for(int i=(p);i<=(k);++i)
#define REP(i,n) FOR(i,0,(n)-1)
#define RFOR(i,p,n) for(int i=(p);i>=(n);--i)
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define ssize(x) int((x).size())
#define fi first
#define se second
#define V vector
#define pb push_back
#define eb emplace_back
#define C const
#define pn printf("\n")
using namespace std;
typedef long long ll;
typedef V<int> vi;
typedef V<ll> vll;
typedef const int ci;
typedef const ll cll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
void chmin(auto &a, auto b){a=min(a,b);}
void chmax(auto &a, auto b){a=max(a,b);}
ci inf = 2.1e9;
cll infll = 4.5e18;

int I(){
    int z;
    scanf("%d", &z);
    //cin >> z;
    return z;
}

void answer(){
    int n = I();
    ll c = I();

    const int cap = 500'000;
    vll dp(cap+1, -infll);
    dp[0] = 0;
    int maks = 0;
    auto akt = [&](int poz, ll wart){
        chmax(dp[poz], wart);
        if(dp[poz] > dp[maks]) maks = poz;
    };

    int ost = 0;
    V<pair<int, ll>> vec;
    FOR(i, 1, n){
        int bok = I(), kol = I();
        if(bok != ost){
            for(auto [poz, wart] : vec) akt(poz, wart);
            vec.clear();
            ost = bok;
        }
        ll wart = bok;
        chmax(wart, dp[kol] + bok);
        chmax(wart, dp[maks] + bok - c);
        vec.eb(kol, wart);
    }
    for(auto [poz, wart] : vec) akt(poz, wart);
    printf("%lld\n", dp[maks]);
}

int main(){
    //ios_base::sync_with_stdio(0);
    //cin.tie(0);
    int tt = 1;
    //tt = I();
    while(tt--) answer();
}