#include <bits/stdc++.h>

using namespace std;

#define REP(i, n) for(int i=0, i##_len=(n); i<i##_len; ++i)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define REP3(i, m, n) for(int i = m, i##_len=(n);i < i##_len;i++)
#define FOREACH(i, v) for(int i = 0;i < v.size();i++)
#define INF 1000000000
#define pb(a) push_back(a)
#define llong long long

int main(){
	int N,D;
	int t,k;
	vector<int> kyoto;
	vector<int> tokyo;
	vector<int> buffer;
	tokyo.pb(0);
	kyoto.pb(0);


	cin>>N>>D;
	REP(i, N){
		cin>>t>>k;
		FOREACH(i,tokyo){
			buffer.pb(tokyo[i]+t);
			buffer.pb(tokyo[i]+k-D);
		}buffer.swap(tokyo);buffer.clear();
		FOREACH(i,kyoto){
			buffer.pb(kyoto[i]+t);
			buffer.pb(kyoto[i]+k-D);
		}buffer.swap(kyoto);buffer.clear();

	}
	int max=0;
	FOREACH(i,tokyo)
		if(tokyo[i]>max)max=tokyo[i];
	FOREACH(i,kyoto)
		if(kyoto[i]>max)max=kyoto[i];

	cout<<max<<endl;
	return 0;
}