#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <queue>
#include <string>
#include <map>
#include <set>
#include <stack>
#include <tuple>
#include <deque>
#include <array>
#include <numeric>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <chrono>
#include <random>
#include <limits>
#include <iterator>
#include <functional>
#include <sstream>
#include <fstream>
#include <complex>
#include <cstring>
#include <unordered_map>
#include <unordered_set>
using namespace std;

using ll = long long;
constexpr int INF = 1001001001;
// constexpr int mod = 1000000007;
constexpr int mod = 998244353;

template<class T>
inline bool chmax(T& x, T y){
    if(x < y){
        x = y;
        return true;
    }
    return false;
}
template<class T>
inline bool chmin(T& x, T y){
    if(x > y){
        x = y;
        return true;
    }
    return false;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int N, D;
    cin >> N >> D;
    vector<vector<int>> dp(N + 1, vector<int>(2, -INF));
    vector<int> reward(2);
    dp[0][0] = 0;
    for(int i = 0; i < N; ++i){
        cin >> reward[0] >> reward[1];
        for(int j = 0; j < 2; ++j){
            if(dp[i][j] == -INF)    continue;
            chmax(dp[i + 1][j], dp[i][j] + reward[j]);
            chmax(dp[i + 1][j ^ 1], dp[i][j] + reward[j ^ 1] - D);
        }
    }
    cout << max(dp[N][0], dp[N][1]) << endl;
    
    return 0;
}