#include<iostream>
#define ll long long

using namespace std;

bool is_prime(ll n){
    if(n == 1) return false;
    if(n == 2 || n == 3 || n == 5) return true;
    if(n % 2 == 0 || n % 3 == 0 || n % 5 == 0){
        return false;
    }
    for(int i=7;i*i <= n;i+=2){
        if(n % i == 0 && n != i) return false;
    }
    return true;
}

int dp[110][2];

int main(){
    int n, d;
    cin >> n >> d;
    int t[n], k[n];
    for (int i = 0; i < n; i++)
    {
        cin >> t[i] >> k[i];
    }

    dp[0][0] = dp[0][1] = 0;
    dp[1][0] = t[0];dp[1][1] = k[0] - d;

    for (int i = 1; i < n; i++)
    {
        dp[i+1][0] = max(dp[i][0]+t[i], dp[i][1]+t[i] - d);
        dp[i+1][1] = max(dp[i][0]+k[i] - d, dp[i][1]+k[i]);
    }

    int ans = max(dp[n][0], dp[n][1]);

    cout << ans << endl;
    return 0;
}