#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <deque>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>

#define CK(N, A, B) (A <= N && N < B)
#define REP(i, a, b) for (int i = a; i < b; i++)
#define RREP(i, a, b) for (int i = (b - 1); a <= i; i--)
#define F first
#define S second
#define ll long long

const int INF = 1e9;
const long long LLINF = 1e18;

using namespace std;

int dy[] = {0,1,0,-1};
int dx[] = {1,0,-1,0};
int dy8[] = {0,1,1,1,0,-1,-1,-1};
int dx8[] = {1,1,0,-1,-1,-1,0,1};

/*==================================================*/
/*variable*/
int N, D;
int T[110], K[110];

ll dp[110][2];  //dp[i][j]: i日目に東京か京都(0 or 1)で仕事をする場合の、収入合計
/*==================================================*/
/*function*/

/*==================================================*/

int main() {
    cin>>N>>D;
    REP(i,0,N){
        cin>>T[i]>>K[i];
    }


    REP(i,0,N){
        REP(j,0,2){
            dp[i][j] = 0;
        }
    }

    dp[0][0] = T[0];
    dp[0][1] = K[0] - D;

    REP(i,0,N-1){
        dp[i+1][0] = max(dp[i][0] + T[i+1], dp[i][1] + T[i+1] - D);
        dp[i+1][1] = max(dp[i][1] + K[i+1], dp[i][0] + K[i+1] - D);
    }

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

    return 0;
}