#include<iostream>
#include<string>
#include<algorithm>
#include<vector>
#include<queue>
#include<map>
#include<math.h>
#include<iomanip>
#include<set>
#include<numeric>
#include<cstring>
#include<cstdio>
#include<functional>
#include<bitset>
#include<limits.h>
#include<cassert>
#include<iterator>
#include<complex>
#include<stack>
using namespace std;
#define lint long long int
/*
vector<char>al(26);
al = {'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
vector<char>AL(26);
AL = {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
*/

int main() {
	lint N, D;
	cin >> N >> D;
	vector<lint>T(N);
	vector<lint>K(N);
	for (int i = 0; i < N; i++) {
		cin >> T[i] >> K[i];
	}
	vector<lint>dp(N);
	dp[0] = max(T[0], K[0] - D);
	for (int i = 1; i < N; i++) {
		if (dp[i - 1] == T[i - 1] || dp[i - 1] == T[i - 1] - D) {
			dp[i] = max(T[i], K[i] - D);
		}
		else if (dp[i - 1] == K[i - 1] || dp[i - 1] == K[i - 1] - D) {
			dp[i] = max(K[i], T[i] - D);
		}
	}
	/*for (int i = 0; i < N; i++) {
		cout << dp[i] << ' ';
	}
	cout << endl;*/
	lint sum = 0;
	for (int i = 0; i < N; i++) {
		sum += dp[i];
	}
	cout << sum << endl;
}