#include<bits/stdc++.h>
#include <boost/multiprecision/cpp_dec_float.hpp>
#include <boost/multiprecision/cpp_int.hpp>
#include <boost/numeric/interval.hpp>
#include <boost/numeric/interval/io.hpp>
using namespace::std;
namespace mp = boost::multiprecision;
typedef mp::number<mp::cpp_dec_float<32,int64_t>> cfloat;
typedef mp::cpp_int cint;
typedef long long lint;
typedef long long ll;
typedef long double ldouble;
typedef vector<lint> vec;
typedef vector<vector<lint>> mat;
typedef vector<vector<vector<lint>>> mat3;
#define rep(i, n) for(lint i = 0; i < (lint)(n); i++)
#define irep(i) for(lint i = 0;; i++)
#define irep1(i) for(lint i = 1;; i++)
#define irep2(i) for(lint i = 2;; i++)
#define rrep(i, n) for(lint i = (lint)(n-1); i >-1; i--)
#define rrepi(i,a,b) for(lint i = (lint)(b-1); i >a-1; i--)
#define repi(i,a,b) for(lint i=lint(a);i<lint(b);i++)
#define rep2(i,a,b,c) for(lint i=lint(a);i>lint(b);i+=c)
#define all(x) (x).begin(),(x).end()
#define sl(c) (('a')<=(c)&&(c)<=('z'))
#define ll(c) (('A')<=(c)&&(c)<=('Z'))
#define max3(a,b,c) max(a,max(b,c))
#define min3(a,b,c) min(a,min(b,c))
#define PI 3.141592653589793
#define cout (cout<<fixed<<setprecision(15))
#define makeupper(t) (transform(all(t),t.begin(),::toupper))
#define makelower(t) (transform(all(t),t.begin(),::tolower))
#define dist(x1,y1,x2,y2) (pow(pow(x2-x1,2)+pow(y2-y1,2),0.5))
#define NEXT_LINE string junk; getline(cin, junk);
#define output(v) do{rep(i,v.size())cout<<(i?" ":"")<<v[i];cout<<"\n";}while(0)
#define output2(v) rep(i,v.size()){rep(j,v[i].size()){cout<<(j?" ":"")<<v[i][j];}cout<<"\n";}
#define INF 1LL<<40
#define IINF 1<<30
#define LINF 9223372036854775807
#define MOD 1000000007
vector<lint> dx={-1,1,0,0,1,1,-1,-1};
vector<lint> dy={0,0,-1,1,1,-1,1,-1};
vector<lint> operator+(const vector<lint>& v1,const vector<lint>& v2){
    vector<lint> v(v1.size()+v2.size(),0);
    rep(i,v1.size())v[i]=v1[i];
    rep(i,v2.size())v[i+v1.size()]=v2[i];
    return v;
}
inline lint sum(vector<lint> v){
    lint sum=0;
    rep(i,v.size())sum+=v[i];
    return sum;
}
inline string replace(string str, string before, std::string after) {
	string::size_type  Pos(str.find(before));
	while (Pos != std::string::npos) {
		str.replace(Pos, before.length(), after);
		Pos = str.find(before, Pos + after.length());
	}
	return str;
}
inline vector<string> split(string s, string delim) {
	vector<string> elems;
	s=replace(s, "#", "HASH");
	s=replace(s, delim,"#");
	stringstream ss(s);
	string item;
	while (getline(ss, item, '#')) {
		elems.push_back(replace(item, "HASH", "#"));
	}
	return elems;
}
inline vector<int> cross(vector<int> a, vector<int> b) {
    return { a[1] * b[2] - a[2] * b[1],a[2] * b[0] - a[0] * b[2],a[0] * b[1] - a[1] * b[0] };
}
inline lint GCD(lint a,lint b){return b?GCD(b,a%b):a;}
inline vector<lint> primeList(lint n) {
    vector<bool> p(n+1);
    vector<lint> list;
    repi(i,1, n+1)p[i] = true;
    repi(i,2,sqrt(n)+1) {
        if (p[i]) {
            repi(j,2,n/i+1) {
                p[i*j] = false;
            }
        }
    }
    repi(i, 2, n+1)if (p[i])list.push_back(i);
    return list;
}
inline set<lint> primeSet(lint n) {
    vector<bool> p(n+1);
    set<lint> set;
    repi(i,1, n+1)p[i] = true;
    repi(i,2,sqrt(n)+1) {
        if (p[i]) {
            repi(j,2,n/i+1) {
                p[i*j] = false;
            }
        }
    }
    repi(i, 2, n+1)if (p[i])set.insert(i);
    return set;
}
inline bool isPrime(int num){
    if (num < 2) return false;
    else if (num == 2) return true;
    else if (num % 2 == 0) return false;
    double sqrtNum = sqrt(num);
    for (int i = 3; i <= sqrtNum; i += 2){
        if (num % i == 0)return false;
    }
    return true;
}
inline lint max(vector<lint> num){
    lint M=num[0];
    rep(i,num.size())M=max(M,num[i]);
    return M;
}
inline lint min(vector<lint> num){
    lint M=num[0];
    rep(i,num.size())M=min(M,num[i]);
    return M;
}
inline cint cmax(vector<cint> num){
    cint M=num[0];
    rep(i,num.size())if(M<num[i])M=num[i];
    return M;
}
inline cint cmin(vector<cint> num){
    cint M=num[0];
    rep(i,num.size())if(M>num[i])M=num[i];
    return M;
}
inline lint signal(lint val){
    if(val==0)return 0;
    else return val/abs(val);
}
struct IVector2{
    lint x,y;
};
bool operator< (const IVector2 &s, const IVector2 &t){
        return s.x==t.x?s.y<t.y:s.x<t.x;
};
bool operator> (const IVector2 &s, const IVector2 &t){
        return s.x==t.x?s.y>t.y:s.x>t.x;
};
struct IVector3{
    lint x,y,z;
};
struct IVector4{
    lint x,y,z,w;
};
struct Tree{
    public:
    vector<map<lint,lint>> list;
    lint n;
    Tree(lint n):n(n){
        list.resize(n);
    }
    void unite(lint a,lint b,lint w){
        list[a].emplace(b,w);
        list[b].emplace(a,w);
    }
    void to(lint a,lint b,lint w){
        list[a].emplace(b,w);
    }
    vec dij(lint x){
        vec _diff(list.size(),INF);
        priority_queue<pair<lint,lint>,vector<pair<lint,lint>>,greater<pair<lint,lint>>>que;
        _diff[x]=0;
        que.push(make_pair(0,x));
        while(!que.empty()){
            lint diff = que.top().first;
            lint to =que.top().second;
            que.pop();
            if(_diff[to]<diff)continue;
            for(auto e:list[to]) {
                if (_diff[e.first] > _diff[to] + e.second) {
                    _diff[e.first] = _diff[to] + e.second;
                    que.push(make_pair(_diff[e.first],e.first));
                }
            }
        }
        return _diff;
    }
};
int main(){
    lint n;
    cin>>n;
    vec t(n),v(n);
    rep(i,n){
        cin>>t[i];
    }
    rep(i,n){
        cin>>v[i];
    }
    lint end_time=sum(t),max_speed=max(v);
    vector<vector<double>> dp(end_time+1,vector<double>(max_speed+1,-1));
    dp[0][0]=0;
    lint now=0;
    lint spend=0;
    repi(i,1,end_time+1){
        rep(j,max_speed+1){
            if(j>v[now])continue;
            double x=-1;
            if(0<=j&&j<=v[now]&&dp[i-1][j]!=-1){
                if(j==v[now])x=max(x,dp[i-1][j]+j);
                else x=max(x,dp[i-1][j]+j+0.25);
            }
            if(0<=j-1&&j-1<=v[now]&&dp[i-1][j-1]!=-1)x=max(x,dp[i-1][j-1]+j-0.5);
            if(0<=j+1&&j+1<=v[now]&&dp[i-1][j+1]!=-1)x=max(x,dp[i-1][j+1]+j+0.5);
            dp[i][j]=x;
        }
        spend++;
        if(spend==t[now]){
            now++;
            spend=0;
        }
    }
    //output2(dp);
    //cout<<dp[60][34];
    cout<<dp[end_time][0];
}

Measured memory_bytes: 8960