#include<iostream>
#include<algorithm>
#include<string>
#include<vector>
#include<iomanip>
#define RREP(i,s,e) for (i = s; i >= e; i--)
#define rrep(i,n) RREP(i,n,0)
#define REP(i,s,e) for (i = s; i < e; i++)
#define rep(i,n) REP(i,0,n)
#define INF 100000000
using namespace std;


int main(){
	int n;
	cin >> n;
	int a[n-1], b[n], c[n];
	int i;
	rep(i,n-1)
		cin >> a[i];

	rep(i,n)
		cin >> b[i] >> c[i];

	int num = -b[0] + c[0];
	int ans = 0;

	REP(i,0,n-1){
		ans += num * a[i];
		num += -b[i+1] + c[i+1];
	}

	cout << ans << endl;

	return 0;
}