#include <iostream>
#include <algorithm>
#include <cstdio>
using namespace std;
typedef long long ll;
ll dp[2][2];
ll n,d;
int main(void){
    scanf("%lld%lld",&n,&d);
    dp[0][1]=-20000000;
    for(int i=0;i<n;i++){
        int u=(i+1)%2;
        ll a,b;
        scanf("%lld%lld",&a,&b);
        dp[u][0]=max(dp[i%2][0]+a,dp[i%2][1]+a-d);
        dp[u][1]=max(dp[i%2][0]+b-d,dp[i%2][1]+b);
    }
    printf("%lld\n",max(dp[n%2][0],dp[n%2][1]));
}