#include <bits/stdc++.h>
using namespace std;

int main(int argc, char const* argv[])
{
	int n;
	cin>>n;
	vector<int> a(n),b(n),c(n);
	int ans=0;
	int zyoukyaku=0;
	for (int i = 0; i < n-1; i++) {
		cin>>a[i];
	}
	for (int i = 0; i < n; i++) {
		cin>>b[i]>>c[i];
		b[0]=c[n]=0;
		zyoukyaku=zyoukyaku+c[n]-b[n];
		if(i>0){
			ans=ans+(a[n-1]*b[n]);
		}
	}
	cout<<ans<<endl;
	return 0;
}