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#include <bits/stdc++.h>
using namespace std;

set<long long> mozliwe[500007];

unordered_map<long long, long long> dp;

int main()
{
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	long long n, c, maxx = 0;
	cin >> n >> c;
	for(long long i = 0; i < n; i++)
	{
		long long a, b;
		cin >> a >> b;
		mozliwe[a].insert(b);
	}
	for(long long i = 500000; i > 0; i--)
	{
		for(auto x : mozliwe[i])
		{
			dp[x] = max(dp[x] + i, maxx - c + i);
		}
		for(auto x : mozliwe[i])
		{
			maxx = max(maxx, dp[x]);
		}
	}
	cout << maxx << endl;
	return 0;
}