#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <map>
#include <stack>
using namespace std;
int n, d;
int x, y;
int field[20][20];
int main() {
    cin >> n >> d;
    int dp[101][2]={0};//0が東京,1が京都
    vector<pair<int, int>> fee(101);
    for(int i=0; i<n; ++i){
        int a, b; cin >> a >> b;
        fee[i].first = a; fee[i].second = b;
    }
    dp[1][0] = fee[0].first; dp[1][1] = fee[0].second-d;
    for(int i=1; i<n; ++i){
        dp[i+1][0] = fee[i].first + max(dp[i][0], dp[i][1]-d);
        dp[i+1][1] = fee[i].second + max(dp[i][0]-d, dp[i][1]);
    }
    cout <<max(dp[n][0], dp[n][1]) << endl;
        
    return 0;
}