#include <algorithm>
#include <cstdlib>
#include <iostream>
#include <vector>
#include <math.h>
using namespace std;
#define int long long

signed main() {
	int N ,D;
	
	cin >> N >> D;
	int T[2][105];
	for (int i = 0; i < N; i++) {
		cin >> T[0][i] >> T[1][i];
	}
	int dp[2][105];
	dp[0][0] = T[0][0];
	dp[1][0] = T[1][0] - D;
	for (int i = 1; i < N; i++) {
		dp[0][i] = max(dp[0][i - 1] + T[0][i], dp[1][i - 1] + T[0][i] - D);
		dp[1][i] = max(dp[1][i - 1] + T[1][i], dp[0][i - 1] + T[1][i] - D);
	}
	cout << max(dp[0][N - 1], dp[1][N - 1]) << endl;
	return 0;
}