#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 p(s) cout<<(s)<<endl
#define REP(i,n,N) for(int i=n;i<N;i++)
#define RREP(i,n,N) for(int i=N-1;i>=n;i--)
#define CK(n,a,b) ((a)<=(n)&&(n)<(b))
#define F first
#define S second
typedef long long ll;
using namespace std;
const ll mod = 1e9+7;

ll N, D;
ll dp[110][2];
ll T[110][2];
int main() {
    cin>>N>>D;
    REP(i,0,N){
        cin>>T[i][0]>>T[i][1];
    }
    dp[0][1] = -D;
    REP(i,0,N){
        REP(j,0,2) {
            dp[i + 1][j] = max(dp[i+1][j], dp[i][j] + T[i][j]);
            dp[i + 1][1-j] = max(dp[i+1][1-j], dp[i][j] + T[i][1-j] - D);
        }
    }

    p(max(dp[N][0], dp[N][1]));

    return 0;
}