#include <iostream>
#include <algorithm>
using namespace std;
const int tokyo = 0;
const int kyoto = 1;
int N, D, T, K;
int dp[100][2];
int main(void) {
	cin >> N >> D;
	cin >> T >> K;
	dp[0][tokyo] = T;
	dp[0][kyoto] = K - D;
	for (int n = 1; n < N; ++n) {
		cin >> T >> K;
		dp[n][tokyo] = max(dp[n - 1][tokyo] + T, dp[n - 1][kyoto] + T - D);
		dp[n][kyoto] = max(dp[n - 1][kyoto] + K, dp[n - 1][tokyo] + K - D);
	}
	cout << max(dp[N - 1][tokyo], dp[N - 1][kyoto]) << endl;
	return 0;
}