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#include<iostream>
#include<iomanip>
#include<vector>
#include<algorithm>

#define REP(i,n) for(int i=0;i<(n);++i)
#define FOR(i,a,b) for(int i=(a);i<=(b);++i)

using namespace std;

typedef long long int llint;

const int MAX_N = 500000;
const int MAX_W = 500000;

int A[MAX_N+1], W[MAX_N+1], T[MAX_N+1];

int n, c, q;

llint best, best0;

struct Seg {
	int d, g, l, r;
	llint v;
};

vector<Seg> ss;

void initSubtree(int d, int g) {
	Seg s;
	s.d = d; s.g = g;
	s.l = s.r = s.v = 0;
	int ix = ss.size();
	ss.push_back(s);
	if (d < g) {
		int m = (d+g)/2;
		ss[ix].l = ss.size();
		initSubtree(d, m);
		ss[ix].r = ss.size();
		initSubtree(m+1, g);
	}
}

void initSeg(int n) {
	Seg s;
	s.d = s.g = s.l = s.r = s.v = -1;
	ss.push_back(s);
	initSubtree(1, n);
}

int immutable_update_at(int w, int ix, llint val) {
	if (!ix) return ix;
	Seg s = ss[ix];
	if (w < s.d || w > s.g) return ix;
	if (s.d == s.g) {
		if (s.v >= val) return ix;
		s.v = val;
		ss.push_back(s);
		return ss.size() - 1;
	} else {
		if (w <= ((s.d + s.g)/2)) {
			int new_ix = immutable_update_at(w, s.l, val);
			if (new_ix == s.l && s.v >= val) return ix;
			s.v = max(s.v, val);
			s.l = new_ix;
			ss.push_back(s);
			return ss.size() - 1;		
		} else {
			int new_ix = immutable_update_at(w, s.r, val);
			if (new_ix == s.r && s.v >= val) return ix;
			s.v = max(s.v, val);
			s.r = new_ix;
			ss.push_back(s);
			return ss.size() - 1;					
		}
	}
}

llint get_max_at_eq(int w, int ix) {
	while(ix) {
		Seg s = ss[ix];
		if (w < s.d || w > s.g) return -1;
		if (s.d == s.g) return s.v;
		ix = (w <= ((s.d + s.g)/2)) ? s.l : s.r;			
	}
	return -1;
}

llint get_max_at_gt(int w, int ix) {
	if (ix == 0) return -1;
	Seg s = ss[ix];
	if (w < s.d) return s.v;
	if (w > s.g) return -1;
	if (s.d == s.g) return s.v;
	return max(get_max_at_gt(w, s.l), get_max_at_gt(w, s.r));
}

llint get_max_at_lt(int w, int ix) {
	if (ix == 0) return -1;
	Seg s = ss[ix];
	if (w < s.d) return -1;
	if (w > s.g) return s.v;
	if (s.d == s.g) return s.v;
	return max(get_max_at_lt(w, s.l), get_max_at_lt(w, s.r));
}

int main(void) {	
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	
	cin >> n >> c;
	FOR(i, 1, n) cin >> A[i] >> W[i];

	initSeg(MAX_W);
	T[0] = 1;
	A[0] = 0;
	q = 0;

	best = 0;
	
	FOR(j, 1, n) {
		if (A[j] != A[j-1]) { q = j-1; }
		
		llint new_v = A[j];
		new_v += max(get_max_at_eq(W[j], T[q]), 
					 max(get_max_at_lt(W[j], T[q]),
					     get_max_at_gt(W[j], T[q])) - c);
		
		T[j] = immutable_update_at(W[j], T[j-1], new_v);
				
		best = max(best, new_v);
	}

	cout << best << endl;
		
	return 0;
}