#include <bits/stdc++.h>
using namespace std;
#define FOR(i, n) for(int i = 0; i < (n); i++)
#define MEM(a, x) memset(a, x, sizeof(a))
#define ALL(a) a.begin(), a.end()
#define UNIQUE(a) a.erase(unique(ALL(a)), a.end())
typedef long long ll;

int n, d;
int dp[105][2];

int main(int argc, char const *argv[]) {
	ios_base::sync_with_stdio(false);
	cin >> n >> d;
	for (int i = 1; i <= n; i++) {
		int c[2];
		cin >> c[0] >> c[1];
		FOR(j, 2) dp[i][j] = max(dp[i-1][j] + c[j], dp[i-1][j^1] + d + c[j^1]);
	}
	cout << (int)max(dp[n][0], dp[n][1]) << endl;
	return 0;
}