#include <iostream>
#include <vector>
#include <algorithm>
#include <queue>

using namespace std;

typedef long long int lli;

struct tower {
    lli a;
    lli score;
};

int main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(nullptr);

    lli n, c, a, w;
    cin >> n >> c;

    tower hi1, hi2;
    hi1.a = 0;
    hi1.score = 0;
    hi2 = hi1;

    vector<tower> patterns(500000 + 1, hi1);
    while ( n-- ) {
        cin >> a >> w;
        lli score = 0;

        score = patterns[w].score;
        if (patterns[w].a < a) score += a;

        if ( hi1.a < a ) {
            score = max(score, a + hi1.score - c);
            if ( score > hi1.score ) {
                hi2 = hi1;
                hi1.a = a;
                hi1.score = score;
            }
        } else {
            score = max(score, a + hi2.score - c);
            if ( score > hi1.score ) {
                hi1.score = score;
                hi1.a = a;
            }  
        }
        patterns[w].a = a; patterns[w].score = score;
    }
    cout << hi1.score << "\n";
}