#include <cmath>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <functional>
#include <queue>
#include <iostream>
#include <string.h>
#include <iomanip>
#include <algorithm>
#include <functional>
#include <cstdint>
#include <climits>
#include <unordered_set>
#include <sstream>
#include <stack>

using namespace std;

typedef long long int ll;
typedef pair<int,int> pii;
typedef tuple<ll,ll,ll> t3;

using namespace std;

vector<ll> dp[2];
vector<ll> ea[2];

int main()
{
    ll n, d;
    cin >> n >> d;
    for(int i = 0;i < 2;i++)
    {
        dp[i].assign(n+10,-1e15);
    }
    for(int i = 0;i < n;i++)
    {
        ll t, k;
        cin >> t >> k;
        ea[0].push_back(t);
        ea[1].push_back(k);
    }
    dp[0][0] = 0;
    for(int i = 0;i < n;i++)
    {
        dp[0][i+1] = max(dp[0][i] + ea[0][i], dp[1][i] + ea[0][i] - d);
        dp[1][i+1] = max(dp[1][i] + ea[1][i], dp[0][i] + ea[1][i] - d);
    }
    ll upper = max(dp[0][n], dp[1][n]);
    cout << upper << endl;
    return 0;
}