#include<bits/stdc++.h>
using namespace std;

#define int long long

#define rep(i,n) for(int i=0;i<(n);i++)
#define pb push_back
#define all(v) (v).begin(),(v).end()
#define fi first
#define se second
typedef vector<int>vint;
typedef pair<int,int>pint;
typedef vector<pint>vpint;

template<typename A,typename B>inline void chmin(A &a,B b){if(a>b)a=b;}
template<typename A,typename B>inline void chmax(A &a,B b){if(a<b)a=b;}

const int INF=1001001001001001001ll;

int N,D;
int T[111],K[111];

int dp[222][2];

signed main(){
    cin>>N>>D;
    rep(i,N)cin>>T[i]>>K[i];

    fill_n(*dp,222*2,-INF);
    dp[0][0]=0;
    rep(i,N){
        chmax(dp[i+1][0],dp[i][0]+T[i]);
        chmax(dp[i+1][0],dp[i][1]+T[i]-D);
        chmax(dp[i+1][1],dp[i][0]+K[i]-D);
        chmax(dp[i+1][1],dp[i][1]+K[i]);
    }
    cout<<max(dp[N][0],dp[N][1])<<endl;
    return 0;
}