#include <bits/stdc++.h>
using namespace std;
int T[100], K[100];
int N, D;

int dp[2][100];

int recursive(int pos, int state) {
	if(pos >= N) return 0;
	if(dp[state][pos] >= 0) return dp[state][pos];
	if(state) {
		dp[state][pos] = K[pos] + max(
			recursive(pos + 1, 0) - D,
			recursive(pos + 1, 1));
	} else {
		dp[state][pos] = T[pos] + max(
			recursive(pos + 1, 1) - D,
			recursive(pos + 1, 0));
	}
	return dp[state][pos];
}

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