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

#define INF 1.1e9
#define LINF 1.1e18
#define FOR(i,a,b) for (int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
#define pb push_back
#define pf push_front
#define fi first
#define se second
#define BIT(x,n) bitset<n>(x)
#define PI 3.14159265358979323846

typedef long long ll;
typedef pair<int,int> P;
typedef pair<int,P> PP;

//-----------------------------------------------------------------------------

int n,a[100],b[100],c[100];

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	cin>>n;
	FOR(i,1,n) cin>>a[i];
	REP(i,n) cin>>b[i]>>c[i];

	int sum=0,now=c[0];
	FOR(i,1,n) {
		sum+=now*a[i];
		now=now-b[i]+c[i];
	}
	cout<<sum<<endl;

	return 0;
}