// UUID: b6041668-da85-498a-8958-1b4a3e8cd78b
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define pll pair<ll,ll>

ll n,k;
vector<pll>tv;

ll cost(ll pos){
	ll sum=0;
	for(int i=0;i<n;i++){
		sum+=(max(tv[i].first-pos,0LL)+max(pos+k-1-tv[i].second,0LL));
	}
	return sum;
}

int main() {
	ios::sync_with_stdio(0);cin.tie(0);
	cin>>n>>k;
	tv.resize(n);
	for(int i=0;i<n;i++){
		cin>>tv[i].first>>tv[i].second;
	}
	ll l=1,r=1e9;
	//cout<<cost(5)<<' '<<l;
	//return 0;
	while(l!=r){
		ll m=(l+r)/2;
		ll c=cost(m);
		ll c2=cost(m+1);
		if(c2>c)r=m;
		if(c2<=c)l=m+1;

	}
	cout<<cost(l);
}