#include<bits//stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define req(i,n) for(int i = 1;i <=  n; i++)
#define rrep(i,n) for(ll i = n-1;i >= 0;i--)
#define ALL(obj) begin(obj), end(obj)
#define RALL(a) rbegin(a),rend(a)
typedef long long int ll;
typedef long double ld;
const int MAX = 510000;
const ll MOD = 1000000007;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
int main(void) {
    int n,d;cin >>n>>d;
    vector<int> t(n),k(n);
    rep(i,n) cin >> t[i] >> k[i];
    vector<vector<int>> dp(2,vector<int>(n+1,0));
    dp[1][0] = -d;
    rep(i,n){
        dp[0][i+1]=max(dp[0][i],dp[1][i]-d)+t[i];
        dp[1][i+1] = max(dp[1][i],dp[0][i]-d)+k[i];
    }cout << max(dp[0][n],dp[1][n]) <<endl;
}