#include <iostream>
#include <algorithm>
using namespace std;
const int N=3e5+1;
int a[N];
int main() {
  long long n,d;
  cin>>n>>d;

int mn=N;
for(int i=0;i<n*2;i++){
  cin>>a[i];
mn=min(mn,a[i]);
}
  int max=0,sum=0;
  for(int i=0;i<n*2;i+=2){
   if((a[i]+a[i+1])<min(a[i],a[i+1])+mn+d)sum+=a[i]+a[i+1];
   else sum+=min(a[i],a[i+1])+mn+d;
   
  }
  cout<<sum;
}