#include<iostream>
#include<cmath>
#include<algorithm>
#include<iomanip>
using namespace std;
float a[100005],b[100005];
float mian(float x,float y,float z)
{
	float o=0;
	o+=(x+y)*z/2;
	return o;
}
int main()
{
	int n;
	float t=0;
	cin>>n;
	for (int i=0;i<n+1;i++)
	{
		cin>>a[i];
	} 
	for (int i=0;i<n;i++)
	{
		cin>>b[i];
	}
	for (int i=0;i<n;i++)
	{
		t+=mian(a[i],a[i+1],b[i]);
	}
	cout<<fixed<<setprecision(1)<<t<<endl;
	return 0;
}