#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, a, b) for(int i = a; i < b; i++)
#define rrep(i, a, b) for(int i = b - 1; i >= a; i--)
#define ALL(a) a.begin(), a.end()
#define pii pair<int,int>
#pragma GCC optimize("Ofast")
#define pcnt __builtin_popcount
#define buli(x) __builtin_popcountll(x)
#define pb push_back
#define mp make_pair
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );
template<class T>bool chmax(T &a, const T &b) {if(a<b){a = b; return 1;} return 0; };
template<class T>bool chmin(T &a, const T &b) {if(a>b){a = b; return 1;} return 0; };
inline void IN(void){return;}
template <typename First, typename... Rest> void IN(First& first, Rest&... rest){cin >> first;IN(rest...);return;}
inline void OUT(void){cout << "\n";return;}
template <typename First, typename... Rest> void OUT(First first, Rest... rest){cout << first << " ";OUT(rest...);return;}
const double EPS = 1e-9;
const int mod = 1e9 + 7;
long long lcm(ll a, ll b){return a * b / __gcd(a,b);}
struct IoSetup { IoSetup() {
    cin.tie(nullptr);ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
} } iosetup;
template< int mod > struct ModInt {
    int x; ModInt() : x(0) {}
    ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}
    ModInt &operator+=(const ModInt &p) {if((x += p.x) >= mod) x -= mod;return *this;}
    ModInt &operator-=(const ModInt &p) {if((x += mod - p.x) >= mod) x -= mod;return *this;}
    ModInt &operator*=(const ModInt &p) {x = (int) (1LL * x * p.x % mod);return *this;}
    ModInt &operator/=(const ModInt &p) {*this *= p.inverse();return *this;}
    ModInt operator-() const { return ModInt(-x); }
    ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }
    ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }
    ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }
    ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }
    bool operator==(const ModInt &p) const { return x == p.x; }
    bool operator!=(const ModInt &p) const { return x != p.x; }
    ModInt inverse() const {int a = x, b = mod, u = 1, v = 0, t;
    while(b > 0) { t = a / b; swap(a -= t * b, b); swap(u -= t * v, v); }return ModInt(u);}
    ModInt pow(int64_t n) const {ModInt ret(1), mul(x); while(n > 0) {if(n & 1) ret *= mul;mul *= mul;n >>= 1;}return ret;}
    friend ostream &operator<<(ostream &os, const ModInt &p) { return os << p.x;}
    friend istream &operator>>(istream &is, ModInt &a) { int64_t t; is >> t; a = ModInt< mod >(t); return (is); }
    static int get_mod() { return mod; }
}; using modint = ModInt< mod >;

void solve(long long N, std::vector<long long> t, std::vector<long long> v){
    vector<ll> tsum(N + 1, 0);
    rep(i, 0, N) tsum[i+1] = tsum[i] + t[i];
    int idx = 1;
    double time = 0, ans = 0, vel = 0;
    while(time < tsum[N]){
        if(time == tsum[idx]) idx++;
        bool flag = 0;
        rep(i, idx, N + 1) if(tsum[i] - time <= vel - v[i+1]) flag = 1;
        //cerr << idx << " " <<  tsum[N] - time << " " <<  vel - v[N+1] << endl;
        if(flag){
            vel -= 0.5;
            ans += (vel*2 + 0.5) / 4.0;
        }else{
            if(vel < v[idx]){ // 加速する場合
                vel += 0.5;
                ans += (vel*2-0.5) / 4.0;
            }else{ // すでに最高速の場合
                ans += 0.5 * vel;
            }
        }
        /* if(vel > v[idx+1]){ // 今の速度が次の制限速度よりも早い
            if(tsum[idx] - time <= vel - v[idx+1]){ //　減速すべき
                vel -= 0.5;
                ans += (vel*2 + 0.5) / 4.0;  
            }else if(vel < v[idx]){ // 加速すべき
                vel += 0.5;
                ans += (vel*2-0.5) * 0.5 * 0.5;
            }else{ // 等速度
                ans += 0.5 * vel;
            }
        }else{ // 遅いから加速してok
            if(vel < v[idx]){ // 加速する場合
                vel += 0.5;
                ans += (vel*2-0.5) / 4.0;
            }else{ // すでに最高速の場合
                ans += 0.5 * vel;
            }
        } */
        time += 0.5;
        /* if((int)time % 10 == 0) */ //cerr << time << " " << vel << " " << ans << endl;
    }
    cout << ans << endl;
}

// Generated by 1.1.6 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
int main(){
    long long N;
    scanf("%lld",&N);
    std::vector<long long> t(N);
    for(int i = 0 ; i < N ; i++){
        scanf("%lld",&t[i]);
    }
    std::vector<long long> v(N+2, 0);
    for(int i = 1 ; i <= N ; i++){
        scanf("%lld",&v[i]);
    }
    solve(N, std::move(t), std::move(v));
    return 0;
}


Measured memory_bytes: 256