#define _CRT_SECURE_NO_WARNINGS
#include "bits/stdc++.h"
#include <random>
//#include "util.k"

using namespace std;

//呪文
#define DUMPOUT cout
#define dump(...) DUMPOUT<<"  ";DUMPOUT<<#__VA_ARGS__<<" :["<<__LINE__<<":"<<__FUNCTION__<<"]"<<endl;DUMPOUT<<"    ";dump_func(__VA_ARGS__)

typedef unsigned uint; typedef long long ll; typedef unsigned long long ull; typedef pair<int, int> pii; typedef pair<ll, ll> pll; typedef pair<double, double> pdd; typedef pair<string, string> pss;
template <typename _KTy, typename _Ty> ostream& operator << (ostream& ostr, const pair<_KTy, _Ty>& m) { ostr << "{" << m.first << ", " << m.second << "}"; return ostr; }
template <typename _KTy, typename _Ty> ostream& operator << (ostream& ostr, const map<_KTy, _Ty>& m) { if (m.empty()) { ostr << "{ }"; return ostr; } ostr << "{" << *m.begin(); for (auto itr = ++m.begin(); itr != m.end(); itr++) { ostr << ", " << *itr; } ostr << "}"; return ostr; }
template <typename _Ty> ostream& operator << (ostream& ostr, const vector<_Ty>& v) { if (v.empty()) { ostr << "{ }"; return ostr; } ostr << "{" << v.front(); for (auto itr = ++v.begin(); itr != v.end(); itr++) { ostr << ", " << *itr; }	ostr << "}"; return ostr; }
template <typename _Ty> ostream& operator << (ostream& ostr, const set<_Ty>& s) { if (s.empty()) { ostr << "{ }"; return ostr; } ostr << "{" << *(s.begin()); for (auto itr = ++s.begin(); itr != s.end(); itr++) { ostr << ", " << *itr; }	ostr << "}"; return ostr; }
template <typename _Ty> ostream& operator << (ostream& ostr, const stack<_Ty>& s) { if (s.empty()) { ostr << "{ }"; return ostr; } stack<_Ty> t(s); ostr << "{" << t.top(); t.pop(); while (!t.empty()) { ostr << ", " << t.top(); t.pop(); } ostr << "}";	return ostr; }
template <typename _Ty> ostream& operator << (ostream& ostr, const list<_Ty>& l) { if (l.empty()) { ostr << "{ }"; return ostr; } ostr << "{" << l.front(); for (auto itr = ++l.begin(); itr != l.end(); ++itr) { ostr << ", " << *itr; } ostr << "}"; return ostr; }
template <typename _KTy, typename _Ty> istream& operator >> (istream& istr, pair<_KTy, _Ty>& m) { istr >> m.first >> m.second; return istr; }
template <typename _Ty> istream& operator >> (istream& istr, vector<_Ty>& v) { for (size_t i = 0; i < v.size(); i++) istr >> v[i]; return istr; }
ll modpow(ll n, ll p, ll m) { ll result = 1; while (p > 0) { if (p & 1) result = (result * n) % m; p >>= 1; n = (n * n) % m; } return result; }
template <typename _Ty> _Ty ipow(_Ty x, _Ty n) { _Ty ret = _Ty(1); for (_Ty i = _Ty(0); i < n; i++) ret *= x; return ret; }
template<typename A, size_t N, typename T> inline void Fill(A(&array)[N], const T &val) { std::fill((T*)array, (T*)(array + N), val); }

void dump_func() { DUMPOUT << endl; }
template <class Head, class... Tail> void dump_func(Head&& head, Tail&&... tail) { DUMPOUT << head; if (sizeof...(Tail) == 0) { DUMPOUT << " "; } else { DUMPOUT << ", "; } dump_func(std::move(tail)...); }



#define PI 3.14159265358979323846
#define EPS 1e-11
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
#define all(x) (x).begin(), (x).end()
#define SZ(x) ((int)(x).size())
#define fake false



double f(double x, int v, int l, int r) {
	if (x <= l) return v + l - x;
	else if (l <= x && x <= r) return v;
	else if (r <= x) return x + v - r;

	return DBL_MIN;
}

int main()
{
	//clock_t start, end;
	//start = clock();

	ios::sync_with_stdio(false);
	cin.tie(0);

	const int INF = 1 << 30;

	int N;
	cin >> N;

	vector<int> t(N + 3);
	vector<int> v(N + 3);

	t[0] = -INF; t[1] = 0;
	for (int i = 2; i <= N + 1; i++) cin >> t[i];
	for (int i = 1; i <= N; i++) t[i + 1] += t[i];
	t[N + 2] = INF;

	v[0] = v[1] = 0;
	for (int i = 2; i <= N + 1; i++) cin >> v[i];
	v[N + 2] = 0;

	vector<pdd> points;
	for (int i = 0; i <= 2 * t[N + 1]; i++) {
		double x = (double)i / 2;
		double y = INF;
		for (int j = 1; j <= N + 2; j++) {
			y = min(y, f(x, v[j], t[j - 1], t[j]));
		}
		points.push_back(pdd(x, y));
	}

	//dump(points);
	double ans = 0;
	for (int i = 0; i < SZ(points) - 1; i++) {
		pdd& p1 = points[i];
		pdd& p2 = points[i + 1];
		double x1 = p1.first, y1 = p1.second;
		double x2 = p2.first, y2 = p2.second;
		
		ans += (y1 + y2) * (x2 - x1) / 2;
	}

	cout << fixed << setprecision(15) << ans << endl;

	//end = clock();
	//printf("%d msec.\n", end - start);

	return 0;
}

Measured memory_bytes: 1396