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#include "bits/stdc++.h"
using namespace std;
#define all(x) x.begin(),x.end()
template<typename A, typename B> ostream& operator<<(ostream &os, const pair<A, B> &p) { return os << p.first << " " << p.second; }
template<typename T_container, typename T = typename enable_if<!is_same<T_container, string>::value, typename T_container::value_type>::type> ostream& operator<<(ostream &os, const T_container &v) { string sep; for (const T &x : v) os << sep << x, sep = " "; return os; }
#ifdef LOCAL
#include "debug.h"
#else
#define debug(...) 42
#define ASSERT(...) 42
#endif
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int,int> pi;
const ll oo = 1e18;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int n,c; cin >> n >> c;
    vi a(n),w(n);
    for(int i=0;i<n;++i) cin >> a[i] >> w[i];
    vector<ll> dp(n);
    const int A = 1 + *max_element(all(w));
    vector<ll> sdp(A);
    ll best=0;

    for(int i=0;i<n;) {
        int j = i;
        while(j<n and a[i]==a[j]) ++j;
        for(int o=i;o<j;++o) {
            dp[o]=max(best+a[o]-c, sdp[w[o]]+a[o]);
        }
        for(int o=i;o<j;++o) {
            best=max(best,dp[o]);
            sdp[w[o]]=max(sdp[w[o]],dp[o]);
        }
        i=j;

    }
    cout<< best << '\n';
    
}