#include<bits/stdc++.h>
#define rep(i,n)for(int i=0;i<(n);i++)
using namespace std;

int dp[200][2];
int main() {
	int n, d; scanf("%d%d", &n, &d);
	dp[0][1] = INT_MIN / 2;
	rep(i, n) {
		int t, k; scanf("%d%d", &t, &k);
		dp[i + 1][0] = max(dp[i][0] + t, dp[i][1] + t - d);
		dp[i + 1][1] = max(dp[i][0] + k - d, dp[i][1] + k);
	}
	printf("%d\n", max(dp[n][0], dp[n][1]));
}