// 基本テンプレート

#include <iostream>
#include <iomanip>
#include <cstdio>
#include <string>
#include <cstring>
#include <deque>
#include <list>
#include <queue>
#include <stack>
#include <vector>
#include <utility>
#include <algorithm>
#include <map>
#include <set>
#include <complex>
#include <cmath>
#include <limits>
#include <cfloat>
#include <climits>
#include <ctime>
#include <cassert>
#include <numeric>
#include <functional>
using namespace std;

#define rep(i,a,n) for(int (i)=(a); (i)<(n); (i)++)
#define repq(i,a,n) for(int (i)=(a); (i)<=(n); (i)++)
#define repr(i,a,n) for(int (i)=(a); (i)>=(n); (i)--)
#define int long long int

template<typename T> void chmax(T &a, T b) {a = max(a, b);}
template<typename T> void chmin(T &a, T b) {a = min(a, b);}
template<typename T> void chadd(T &a, T b) {a = a + b;}

typedef pair<int, int> pii;
typedef long long ll;

int dx[] = {0, 0, 1, -1};
int dy[] = {1, -1, 0, 0};
constexpr ll INF = 1001001001001001LL;
constexpr ll MOD = 1000000007LL;

int N, D;
int dp[110][2];
int job[110][2];

signed main() {
    cin >> N >> D;
    rep(i,0,N) {
        cin >> job[i][0] >> job[i][1];
    }

    memset(dp, -1, sizeof(dp));
    dp[0][0] = 0;
    rep(i,0,N) rep(j,0,2) {
        if(dp[i][j] < 0) continue;
        rep(k,0,2) {
            if(j == k) chmax(dp[i+1][k], dp[i][j] + job[i][k]);
            else chmax(dp[i+1][k], dp[i][j] + job[i][k] - D);
        }
    }
    cout << max(dp[N][0], dp[N][1]) << endl;
    return 0;
}