#include<bits/stdc++.h>
using namespace std;
int a[1000005],b[1000005];
int sum;
int main(){
	int n,m;
	cin>>n>>m;
	for(int i=1;i<=n;i++){
		cin>>a[i]>>b[i];
	}
	sum+=a[1]*b[1];
	for(int i=2;i<=n;i++){
		int minn=a[i];
		int jiage=0;
		int ans;
		for(int j=i-1;j>=1;j--){
			ans=(m*i-j)*b[i]; 
			if(a[j]+ans<minn){
				jiage=minn*b[i];
				minn=min(minn,a[j]+ans);
			}
		}
		sum+=jiage;
	}
	cout<<sum;
	return 0;
}