#include <bits/stdc++.h>
#define rep(i,a,b) for(int i=a;i<b;i++)
#define rrep(i,a,b) for(int i=a;i>=b;i--)
using namespace std;
typedef long long ll;
typedef pair<int, int> P;

int main(){
    const ll INF = 1e18;
    int N;
    ll D;
    cin >> N >> D;

    ll dp[N+1][2];
    rep(i,0,N+1) rep(j,0,2) dp[i][j] = 0;

    ll salary[N+1][2];
    rep(i,1,N+1) cin >> salary[i][0] >> salary[i][1];

    dp[1][0] = salary[1][0];
    dp[1][1] = salary[1][1] - D;

    rep(i,2,N+1) rep(j,0,2){
        if(j==0) dp[i][j] = max(dp[i-1][1]+salary[i][0]-D, dp[i-1][0]+salary[i][0]);
        else dp[i][j] = max(dp[i-1][0]+salary[i][1]-D, dp[i-1][1]+salary[i][1]);
    }
    cout << max(dp[N][0], dp[N][1]) << endl;
}