#include <bits/stdc++.h>
#define ll long long
#define INF 1000000005
#define MOD 1000000007
#define EPS 1e-10
#define rep(i,n) for(int i=0;i<(int)n;++i)
#define each(a,b) for(auto (a): (b))
#define all(v) (v).begin(),(v).end()
#define zip(v) sort(all(v)),v.erase(unique(all(v)),v.end())
#define fi first
#define se second
#define pb push_back
#define show(x) cout<<#x<<" = "<<(x)<<endl
#define spair(p) cout<<#p<<": "<<p.fi<<" "<<p.se<<endl
#define svec(v) cout<<#v<<":";rep(kbrni,v.size())cout<<" "<<v[kbrni];cout<<endl
#define sset(s) cout<<#s<<":";each(kbrni,s)cout<<" "<<kbrni;cout<<endl
#define smap(m) cout<<#m<<":";each(kbrni,m)cout<<" {"<<kbrni.first<<":"<<kbrni.second<<"}";cout<<endl

using namespace std;

typedef pair<int,int>P;

const int MAX_N = 105;

int dp[MAX_N][2];

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n,d;
    cin >> n >> d;
    vector<int> u(n),v(n);
    rep(i,n){
        cin >> u[i] >> v[i];
    }
    rep(i,n+1){
        dp[i][0] = dp[i][1] = 0;
    }
    dp[0][1] = -INF;
    rep(i,n){
        dp[i+1][0] = max(dp[i+1][0],dp[i][0]+u[i]);
        dp[i+1][0] = max(dp[i+1][0],dp[i][1]+u[i]-d);
        dp[i+1][1] = max(dp[i+1][1],dp[i][1]+v[i]);
        dp[i+1][1] = max(dp[i+1][1],dp[i][0]+v[i]-d);
    }
    cout << max(dp[n][0],dp[n][1]) << endl;
    return 0;
}