#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

int main() {
    int n, d;
    cin >> n >> d;
    int t[n], k[n];
    rep(i,n)cin >> t[i] >> k[i];

    int dp[n][2];
    dp[0][0] = t[0];
    dp[0][1] = k[0] - d;

    srep(i,1,n){
        dp[i][0] = max(dp[i-1][0] + t[i], dp[i-1][1] - d + t[i]);
        dp[i][1] = max(dp[i-1][0] - d + k[i], dp[i-1][1] + k[i]);
    }

    int ans = max(dp[n-1][0], dp[n-1][1]);

    cout << ans << endl;
    return 0;
}