#include <bits/stdc++.h>
#define rep(i,n)for(int i=0;i<(n);i++)
using namespace std;

int a[200];
int main(){
	int n;scanf("%d",&n);
	rep(i,n-1)scanf("%d",&a[i]);
	int cnt=0,ans=0;
	rep(i,n){
		int b,c;scanf("%d%d",&b,&c);
		cnt+=c-b;
		ans+=cnt*a[i];
	}
	cout<<ans<<endl;
}