#include <bits/stdc++.h>

using i64 = long long;
using u64 = unsigned long long;
using u32 = unsigned;
using u128 = unsigned __int128;

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    
    int n, c;
    std::cin >> n >> c;
    
    std::vector<int> a(n), w(n);
    for (int i = 0; i < n; i++) {
        std::cin >> a[i] >> w[i];
    }
    
    int m = *std::max_element(w.begin(), w.end()) + 1;
    
    std::vector<i64> dp(m);
    
    i64 mx = 0;
    
    std::vector<std::pair<int, i64>> tmp;
    for (int l = 0, r = 0; l < n; l = r) {
        while (r < n && a[l] == a[r]) {
            r++;
        }
        
        for (int i = l; i < r; i++) {
            tmp.push_back({w[i], std::max(dp[w[i]] + a[i], mx + a[i] - c)});
        }
        for (auto [x, v] : tmp) {
            dp[x] = std::max(dp[x], v);
            mx = std::max(mx, dp[x]);
        }
        tmp.clear();
    }
    
    std::cout << mx << "\n";
    
    return 0;
}