#include "bits/stdc++.h"

#define REP(i, n, N) for(ll i=(n); i<(N); i++)
#define RREP(i, n, N) for(ll i=(N-1); i>=(n); i--)
#define LREP(lst,itr) for(auto itr = lst.begin(); itr != lst.end(); ++itr)
#define SPACEREP(lst,itr) for(auto itr=lst.begin(); itr!=lst.end();++itr){cout<<*itr<<(++itr!=anslist.end()?" ":"\n");itr--;}
#define CK(n, a, b) ((a)<=(n)&&(n)<(b))
#define ALL(v) (v).begin(),(v).end()
#define MCP(a, b) memcpy(b,a,sizeof(b))
#define P(s) cout<<(s)<<endl
#define P2(a, b) cout<<(a)<<" "<<(b)<<endl
#define V2(T) vector<vector<T>>
typedef long long ll;
using namespace std;
const ll MOD = 1e9 + 7;
const ll INF = 1e18;

ll dp[101][2];

int main(){
    ll n,d,tmp;
    cin >> n >> d;
    vector<vector<ll>> input(n+1,vector<ll>(2));
    REP(i,1,n+1){
        REP(j,0,2){
            cin >> tmp;
            input[i][j]=tmp;
        }
    }

    dp[1][0]=input[1][0];
    dp[1][1]=input[1][1]-d;

    REP(i,1,n){
        dp[i+1][0] = max(dp[i][0]+input[i+1][0],dp[i][1]+input[i+1][0]-d);
        dp[i+1][1] = max(dp[i][0]+input[i+1][1]-d,dp[i][1]+input[i+1][1]);
    }
    cout << max(dp[n][0],dp[n][1]) <<endl;

}