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#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>
using namespace std;

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define FORD(i,a,b) for(int i=(b)-1;i>=(a);--i)
#define REP(i,n) FOR(i,0,n)
#define REPD(i,n) FORD(i,0,n)
#define VAR(v,w) __typeof(w) v=(w)
#define FORE(it,c) for(VAR(it,(c).begin());it!=(c).end();++it)
#define PB push_back
#define ALL(c) (c).begin(),(c).end()
#define SIZE(c) ((int)(c).size())
#define MP make_pair
#define FT first
#define SD second
#define CLEAR(a) memset((a), 0, sizeof(a))
#define INT(x) int x; scanf("%d", &x)
#define LLG(x) LL x; scanf("%lld", &x)
#define STR(n,x) char x[n]; scanf("%s", x)
#define LINE(n,x) char x[n]; fgets(x, n, stdin)

typedef long long LL;
typedef unsigned long long ULL;
typedef long double LD;
typedef pair<int,int> PII;
typedef vector<int> VI;
typedef vector<string> VS;
typedef vector<VI> VVI;
typedef vector<PII> VII;
typedef vector<double> VD;
typedef vector<LD> VLD;
typedef vector<LL> VLL;
typedef vector<bool> VB;

PII b[500000];
LL r[500001], h;

int main() {
	INT(n);
	INT(c);
	REP(i,n) scanf("%d%d", &b[i].FT, &b[i].SD);
	sort(b, b + n, greater<PII>());
	int i = 0;
	while (i < n) {
		int j = i + 1;
		while (j < n && b[i].FT == b[j].FT) ++j;
		map<int, LL> p;
		FOR(k,i,j) p[b[k].SD] = max(p[b[k].SD], b[k].FT + max(r[b[k].SD], h - c));
		FORE(it,p) {
			r[it->FT] = max(r[it->FT], it->SD);
			h = max(h, it->SD);
		}
		i = j;
	}
	printf("%lld\n", h);
}