// Slower C++ solution:
#include <vector>

#include <iostream>

#include <cmath>

#include <map>

#include <unordered_map>

#include <algorithm>

#include <fstream>

#include <unistd.h>

#include <string>

#include <numeric>

#include <queue>

#include <deque>

#include <sstream>

#include <iomanip>

#include <set>

#include <stack>

#include <cassert>

using namespace std;

typedef long long ll;

typedef vector<int> vi;

typedef vector<ll> vl;

typedef vector<vl> vvl;

typedef vector<vvl> vvvl;

typedef vector<vi> vvi;

typedef vector<vvi> vvvi;

typedef vector<bool> vb;

typedef vector<vector<bool>> vvb;

typedef vector<string> vs;

typedef vector<vector<string>> vvs;

typedef vector<char> vc;

typedef vector<vector<char>> vvc;

typedef vector<double> vd;

typedef vector<vd> vvd;

typedef pair<int, int> P;

typedef pair<ll, ll> PL;

typedef vector<P> vp;

typedef vector<PL> vpl;

typedef vector<vector<P>> vvp;

typedef vector<vector<PL>> vvpl;

const int INF = 1001001001;

const ll LINF = 1e10;

const double pi = 3.1415926535897932;

const string endstr = "\n";

#define FOR(i, a, b) for(ll i = (a); i < b; i++)

#define REP(i, n) for(ll i = 0; i < n; i++)

#define FORMAP(it, m) for(auto it = m.begin(); it != m.end(); it++)

#define ff first

#define ss second

#define pb push_back



template <typename T>

T gcd(T a, T b) {

    return (a == 0) ? b : gcd(b%a, a);

}

template <typename T>

T lcm(T a, T b) {

    return a / gcd(a, b) * b;

}



bool p_comp_fs(const PL p1, const PL p2){ return p1.first < p2.first;};

bool p_comp_fg(const PL p1, const PL p2){ return p1.first > p2.first;};

bool p_comp_ss(const PL p1, const PL p2){ return p1.second < p2.second;};

bool p_comp_sg(const PL p1, const PL p2){ return p1.second > p2.second;};

template <typename T>

vector<T> uniquen(vector<T> vec){

    vec.erase(unique(vec.begin(), vec.end()), vec.end());

    return vec;

}





int main(){

    ll N; cin >> N;

    vl t(110, 0), s(110, 0);

    FOR(i,1, N+1){

        cin >> t[i]; t[i] *= 2;

        s[i+1] = s[i] + t[i];

    }

    vl v(110, 0);

    FOR(i,1, N+1){

        cin >> v[i];

        v[i] *= 2;

    }

    

    vl p(40100, 0);

    FOR(i, 0, s[N+1] + 1){

        FOR(j, 1, N+2){

            if(i <= s[j]){

                p[i] = v[j-1]; break;

            }

        }

        FOR(j, 1, N+2){

            ll dst = abs(s[j] - i);

            p[i] = min(p[i], dst + min(v[j-1],v[j]));

        }

    }

    

    ll ans = 0;

    REP(i, s[N+1]+1){

        ans += p[i];

    }

    double ansd = (double)0.25 * ans;

    cout << fixed << std::setprecision(20) << ansd << endl;

    return 0;

}


// Faster C++ solution for the same problem (measured speedup: 1.3160299135424651x):
#include <vector>

#include <iostream>

#include <cmath>

#include <map>

#include <algorithm>

#include <fstream>

#include <unistd.h>

#include <string>

#include <numeric>

#include <queue>

#include <deque>

#include <sstream>

#include <iomanip>

#include <set>

#include <sys/time.h>

#include <sys/resource.h>

#include <thread>

#include <chrono>

using namespace std;

typedef long long ll;

typedef vector<int> vi;

typedef vector<ll> vl;

typedef vector<vl> vvl;

typedef vector<vi> vvi;

typedef vector<vvi> vvvi;

typedef vector<bool> vb;

typedef vector<vector<bool>> vvb;

typedef vector<string> vs;

typedef vector<vector<string>> vvs;

typedef vector<char> vc;

typedef vector<vector<char>> vvc;

typedef pair<int, int> P;

typedef pair<ll, ll> PL;

typedef vector<P> vp;

typedef vector<PL> vpl;

typedef vector<vector<P>> vvp;

typedef vector<vector<PL>> vvpl;

const int INF = 1001001001;

const ll LINF = 1e18;

const double pi = 3.1415926535897932;

const string endstr = "\n";

#define FOR(i, a, b) for(ll i = (a); i < b; i++)

#define rep(i, n) for(ll i = 0; i < n; i++)

#define rrep(i, n) for(ll i = n; i >= 0; i--)

#define FORMAP(it, m) for(auto it = m.begin(); it != m.end(); it++)

#define ff first

#define ss second

#define pb push_back



template <typename T>

T gcd(T a, T b) {

    return (a == 0) ? b : gcd(b%a, a);

}

template <typename T>

T lcm(T a, T b) {

    return a / gcd(a, b) * b;

}



bool p_comp_fs(const PL p1, const PL p2){ return p1.first < p2.first;};

bool p_comp_fg(const PL p1, const PL p2){ return p1.first > p2.first;};

bool p_comp_ss(const PL p1, const PL p2){ return p1.second < p2.second;};

bool p_comp_sg(const PL p1, const PL p2){ return p1.second > p2.second;};

bool pcomp(const PL p1, const PL p2){ return p1.ss*p2.ff < p2.ss*p1.ff;};

template <typename T>

vector<T> uniquen(vector<T> vec){

    vec.erase(unique(vec.begin(), vec.end()), vec.end());

    return vec;

}



template<class T> bool chmax(T &a, T b) { if (a < b) { a = b; return true; } return false; }

template<class T> bool chmin(T &a, T b) { if (a > b) { a = b; return true; } return false; }



struct stc {ll l, r, v;};



int main(){

    ll N; cin >> N;

    vl T(N+3); T[0] = -1; T[1] = 0;

    rep(i, N){

        cin >> T[i+2]; T[i+2] *= 2; T[i+2] += T[i+1];

    }

    T[N+2] = T[N+1] + 1;

    vl V(N+2, 0);

    rep(i, N){

        cin >> V[i+1]; V[i+1] *= 2;

    }

    

    vector<stc> vs(N+2);

    rep(i, N+2){

        vs[i] = {T[i], T[i+1], V[i]};

    }

    

    vl ans(T[N+1]+1, 0);

    rep(now, T[N+1]+1){

        ll mn = LINF;

        rep(i, N+2){

            auto st = vs[i];

            if(now <= st.l){

                chmin(mn, st.v + abs(now-st.l));

            }

            else if(st.r <= now){

                chmin(mn, st.v + abs(now-st.r));

            }

            else{

                chmin(mn, st.v);

            }

        }

        ans[now] = mn;

    }

    

    double res = 0.0;

    rep(i, T[N+1]){

//        cout << i << " " << ans[i] << endstr;

        res += (double)(ans[i]+ans[i+1])/2.0 / 4.0;

    }

    

    cout << fixed << setprecision(20) << res << endstr;

    return 0;

}