#include <bits/stdc++.h>
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );
using namespace std;
using ll = long long;

int dp[101][2];

int main(){
    int n, d;
    cin >> n >> d;
    vector<int> v(n), u(n);
    rep(i, n) cin >> v[i] >> u[i];

    dp[0][1] = -d;

    rep(i, n){
        dp[i+1][0] = max({dp[i+1][0], dp[i][0]+v[i], dp[i][1]+v[i]-d});
        dp[i+1][1] = max({dp[i+1][1], dp[i][1]+u[i], dp[i][0]+u[i]-d});
    }

    cout << max(dp[n][0], dp[n][1]) << endl;
}