#include <iostream>
#include <vector>
using namespace std;

int main() {
	int D, T, K, tmp = 0;
	size_t N;
	cin >> N >> D;
	vector<int> dp(N + 1);
	dp[0] = 0;
	for (size_t i = 0; i < N; ++i) {
		dp[i + 1] = dp[i];
		cin >> T >> K;
		if (tmp % 2) {
			if (T - D > K) {
				dp[i + 1] += T - D;
				++tmp;
			}
			else dp[i + 1] += K;
		}
		else {
			if (T > K - D) dp[i + 1] += T;
			else {
				dp[i + 1] += K - D;
				++tmp;
			}
		}
	}
	cout << dp[N];
}