#include <bits/stdc++.h>
#define REP(i,n,N) for(int i=(n);i<(int) N;i++)
#define RREP(i,n,N) for(int i=N-1;i>=(int) n;i--)
#define p(s) cout<<(s)<<endl
#define DEBUG(x,y) cout<<#x<<": "<<x<<" , "<<#y<<": "<<y<<endl;
#define CK(n,a,b) ((a)<=(n)&&(n)<(b))
#define F first
#define S second
typedef long long ll;
using namespace std;
const ll mod=1e9+7;

ll dp[100][2];
int main(){
	ll N,D;
	cin>>N>>D;
	dp[0][1]=-D;
	REP(i,0,N){
		ll T,K;
		cin>>T>>K;
		dp[i+1][0]=max(dp[i][0]+T,dp[i][1]+K-D);
		dp[i+1][1]=max(dp[i][0]+T-D,dp[i][1]+K);
	}
	p(max<ll>(dp[N][0],dp[N][1]));
	return 0;
}