#include<bits/stdc++.h>
#define rep(i,n,m) for(int i = (n); i <(m); i++)
#define rrep(i,n,m) for(int i = (n) - 1; i >=(m); i--)
#define chmax(a, b) a = max((a), (b))
using namespace std;
using ll = long long;
const int INF = 1000000000;


int main()
{
    int n, d;
    cin >> n >> d;
    vector<int> tokyo(n), kyoto(n);
    rep(i, 0, n) cin >> tokyo[i] >> kyoto[i];

    vector<vector<int>> dp(n+1, vector<int>(2, -INF));
    dp[0][0] = 0;
    // 0 -> tokyo , 1 -> kyoto
    rep(i, 0, n)
    {
        // from tokyo 
        chmax(dp[i+1][0], dp[i][0]+tokyo[i]);
        chmax(dp[i+1][1], dp[i][0]+kyoto[i]-d);
        // from kyoto
        chmax(dp[i+1][0], dp[i][1]+tokyo[i]-d);
        chmax(dp[i+1][1], dp[i][1]+kyoto[i]);
    }
    cout << max(dp[n][0], dp[n][1]) << endl;
    return 0;
}