#include<iostream>
#include <fstream>
#include <cmath>
#include <algorithm>
#include <stack>
#include <vector>
#include <random>
#include <unordered_set>
using namespace std;
typedef long long ll;
#define FOR(I,A,B) for(int I = int(A); I < int(B); ++I)

int main(){
    ll n,d;
    cin >> n >> d;
    vector<ll> T(n),K(n);
    FOR(i,0,n) cin >> T[i] >> K[i];
    
    ll dp[n][2];
    dp[0][0] = T[0];
    dp[0][1] = K[0]-d;
    
    FOR(i,1,n){
        dp[i][0] = max(dp[i-1][0]+T[i],dp[i-1][1]+T[i]-d);
        dp[i][1] = max(dp[i-1][0]+K[i]-d,dp[i-1][1]+K[i]);
    }
    
    cout << max(dp[n-1][0],dp[n-1][1]) << endl;
}