#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=a;i<b;i++)
#define FORS(i,a,b) for(int i=a+1;i<=b;i++)
#define REP(i,N) FOR(i,0,N)
#define REPS(i,N) FORS(i,0,N)
#define debug(x) cout<<#x<<"="<<x<<endl
#define debugarr(arr,N,M){REP(i,N){REP(j,M){if(arr[i][j]==INF)printf(" INF");\
	else printf(" %3d",arr[i][j]);} cout<<endl;}}
#define pb push_back
#define pob pop_back
typedef long long ll;
const int INF=100000000;
//--------------------------------------//

int N,D;
int T[100],K[100];

int dp[101];
int main(){
	cin>>N>>D;
	REP(i,N) cin>>T[i]>>K[i];
	bool nowcity=true; //trueが東京
	dp[0]=0;
	REP(i,N){
		if(nowcity){
			if(T[i]==K[i]-D&&i!=N-1){
				dp[i+1]=dp[i]+T[i];
				nowcity=(T[i+1]>K[i+1]?true:false);
			}
			else if(T[i]>K[i]-D) dp[i+1]=dp[i]+T[i];
			else{
				dp[i+1]=dp[i]+K[i]-D;
				nowcity=false;
			}
		}
		else{
			if(K[i]==T[i]-D&&i!=N-1){
				dp[i+1]=dp[i]+K[i];
				nowcity=(T[i+1]>K[i+1]?true:false);
			}
			else if(K[i]>T[i]-D) dp[i+1]=dp[i]+K[i];
			else{
				dp[i+1]=dp[i]+T[i]-D;
				nowcity=true;
			}
		}
	}
	REP(i,N+1) debug(dp[i]);
	cout<<dp[N]<<endl;
	return 0;
}