#include <bits/stdc++.h>

using namespace std;
using ll = long long;

int main(){
    int n , d;
    cin >> n >> d;
    vector<vector<ll>> dp(n + 10 , vector<ll>(2 , 0));
    for(int i = 0; i < n; i++){
        ll a , b;
        cin >> a >> b;
        if(i == 0){
            dp[0][0] = a;
            dp[0][1] = b - d;
        }else{
            dp[i][0] = max(dp[i - 1][0] + a , dp[i - 1][1] + a - d);
            dp[i][1] = max(dp[i - 1][1] + b , dp[i - 1][0] + b - d);
        }
    }
    cout << max(dp[n - 1][0] , dp[n - 1][1]) << endl;
    return 0;
}