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#include <cstdio>
#include <cstdlib>

#include <algorithm>
#include <vector>
#include <set>

using lli = long long int;

int main() {
    int n;
    lli c;
    scanf("%d %lld\n", &n, &c);

    std::vector<lli> best_per_end_pattern;
    best_per_end_pattern.resize(500 * 1000, 0);

    lli best_overall = 0;

    // Size of the previously/currently processed block
    lli current_size = 0;

    // Contains the pattern IDs for each block seen so far that has the same
    // size as the current block
    std::set<int> current_group;

    auto flush_group = [&] () {
        const lli best = best_overall;
        for (int w : current_group) {
            best_per_end_pattern[w] = std::max(
                    best_per_end_pattern[w] + current_size,
                    best + current_size - c);
            
            best_overall = std::max(best_overall, best_per_end_pattern[w]);
        }
        current_group.clear();
    };

    for (int i = 0; i < n; i++) {
        lli a;
        int w;
        scanf("%lld %d\n", &a, &w);
        w--; // We use zero-based indexing
        if (current_size != a) {
            flush_group();
            current_size = a;
        }
        current_group.insert(w);
    }

    flush_group();

    printf("%lld\n", best_overall);

    return 0;
}