#include <iostream>
#include <iomanip>
#include <cstring>
#include <algorithm> 
#include <queue>
#include <functional>
#include <map>
#include <vector>
#include <string>
using namespace std;
typedef long long ll;

ll dp[105][2];
ll T[105], K[105];
int n,d;
int main(void){
    cin >> n >> d;
    for (int i = 0; i < n; i++){
        cin >> T[i] >> K[i];
    }

    dp[0][0] = 0;
    dp[0][1] = -100000000;

    for (int i = 0; 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][1] + K[i], dp[i][0] + K[i] - d);
    }

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

}