#include <bits/stdc++.h>
using namespace std;

//#include <boost/multiprecision/cpp_int.hpp>
// using namespace boost::multiprecision;
// using cint = cpp_int;
//#pragma GCC optimize("", on)
//#pragma GCC optimization_level 3

// Define
using ll = long long;
using ull = unsigned long long;
using ld = long double;
const ll MOD = 1e9 + 7;
const ll INF = LONG_MAX;
const ull MAX = ULONG_MAX;
#define mp make_pair
#define pb push_back
#define eb emplace_back
#define x first
#define y second
#define endl '\n'
#define space ' '
#define def inline auto
#define func inline constexpr ll
#define run __attribute__((constructor)) def _
#define all(v) begin(v), end(v)
#define input(a) scanf("%lld", &(a))
#define print(a) printf("%lld\n", (a))

// Debug
#define debug(...)                                                             \
    {                                                                          \
        cerr << __LINE__ << ": " << #__VA_ARGS__ << " = ";                     \
        for (auto &&X : {__VA_ARGS__}) cerr << "[" << X << "] ";               \
        cerr << endl;                                                          \
    }

#define dump(a, h, w)                                                          \
    {                                                                          \
        cerr << __LINE__ << ": " << #a << " = [" << endl;                      \
        rep(i, h) {                                                            \
            rep(j, w) cerr << a[i][j] << space;                                \
            cerr << endl;                                                      \
        }                                                                      \
        cerr << "]" << endl;                                                   \
    }

#define lldump(a, n) dump(vector<vector<ll>>{a}, 1, n)

// Loop
#define inc(i, a, n) for (ll i = (a), _##i = (n); i <= _##i; ++i)
#define dec(i, a, n) for (ll i = (a), _##i = (n); i >= _##i; --i)
#define each(i, a) for (auto &&i : a)
#define rep(i, n) inc(i, 0, n - 1)
#define loop() for (;;)

// Stream
#define fout(n) cout << fixed << setprecision(n)
#define fasten cin.tie(0), ios::sync_with_stdio(0)

// Speed
run() { fasten, fout(10); }
#pragma GCC optimize("O3")
#pragma GCC target("avx")

// Math
func gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
func lcm(ll a, ll b) { return a * b / gcd(a, b); }
func sign(ll a) { return a ? abs(a) / a : 0; }

def in() {
    ll A;
    cin >> A;
    return A;
}

signed main() {
    ll N;
    double res = 0;
    cin >> N;
    ll T[N], V[N];
    rep(i, N) cin >> T[i];
    rep(i, N) cin >> V[i];
    ll M = accumulate(T, T + N, 0LL), R = 0;
    double S[2 * M + 1][N + 2];
    rep(j, N) {
        rep(i, 2 * M + 1) {
            double t = double(i) / 2;
            if (t < R)
                S[i][j] = V[j] + (R - t);
            else if (R + T[j] < t)
                S[i][j] = V[j] + (t - (R + T[j]));
            else
                S[i][j] = V[j];
        }
        R += T[j];
    }
    rep(i, 2 * M + 1) S[i][N] = double(i) / 2;
    rep(i, 2 * M + 1) S[i][N + 1] = double(2 * M - i) / 2;
    rep(i, 2 * M + 1) S[i][0] = *min_element(S[i], S[i] + N + 2);
    rep(i, 2 * M) res += (S[i][0] + S[i + 1][0]) * 0.25;
    cout << res << endl;
}

// for compilation: g++ -Ofast -march=native -o _ _.cpp -std=c++17


Measured memory_bytes: 32128