#include<iostream>
using namespace std;
int n,a[100],cnt,b,c;
main()
{
	cin>>n;
	for(int i=1;i<n;i++)cin>>a[i];
	cin>>b>>cnt;
	int ans=0;
	for(int i=1;i<n;i++)
	{
		ans+=a[i]*cnt;
		cin>>b>>c;
		cnt+=c-b;
	}
	cout<<ans<<endl;
}