#include <iostream>
#include <iostream>
#include <vector>
using namespace std;
int main(int argc, char* argv[])
{
	int n;
	cin>>n;
	vector<int> A1(n);
	int i,j,k;
	for (i=1;i<n;i++){
		cin>>A1[i];
	}
	vector<int> B(n+1),C(n+1),D(n+1);
	for (i=1;i<=n;i++){
		cin>>B[i]>>C[i];
	}
	int aw=0;
	j=1;
	for (i=2;i<=n;i++){
		while (j<n && C[j]==0){
			j++;
		}
		int a=0;
		for (k=j;k<i;k++){
			a+=A1[k];
		}
		if (C[j]>=B[i]){
			aw+=a*B[i];
			C[j]-=B[i];
			B[i]=0;
		}else{
			aw+=a*C[j];
			B[i]-=C[j];
			C[j]=0;
			j++;
			i--;
		}
	}	
	cout<<aw<<endl;
	return 0;
}