import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.lang.reflect.Array;
import java.util.*;

public class Main {
    static InputStream is;
    static PrintWriter out;
    static String INPUT = "";

    void solve() {
        int n = ni();
        int t[] = new int[n + 2];
        int v[] = new int[n + 2];
        for (int i = 0; i < n; ++i) t[i + 1] = ni();
        for (int i = 0; i < n; ++i) v[i + 1] = ni();
        for (int i = n; i > 0; --i) {
            v[i] = Math.min(v[i], v[i + 1] + t[i]);
        }
        for (int i = 1; i <= n; ++i) {
            v[i] = Math.min(v[i], v[i - 1] + t[i]);
        }
        double x = 0;
        for (int i = 0; i < n; ++i) {
            x += calc(Math.min(v[i], v[i + 1]), v[i + 2], t[i + 1], v[i + 1]);
        }
        out.println(x);
    }

    double calc(int s, int e, int t, int v) {
        double m = 0;
        if (e > v) e = v;
        for (int i = 0; i <= t * 2; ++i) {
            for (int j = i; j <= t * 2; ++j) {
                int r = j - i;
                int k = t * 2 - j;
                if ((v - s) * 2 < i) continue;
                if ((e - s) * 2 < i - k) continue;
                if (-s * 2 > i - k) continue;
                double b = i / 2.0;
                double p = 0;
                p += s * b + b * b / 2.0;
                p += (s + b) * (r / 2.0);
                b = k / 2.0;
                p += (s + i / 2.0 - k / 2.0) * b + b * b / 2.0;
                // tr(i, r, k, p, s * b + b * b / 2.0, (s + b) * (j / 2.0), e * b + b * b / 2.0);
                if (m < p) m = p;
            }
        }
        return m;
    }

    public static void main(String[] args) throws Exception {
        long S = System.currentTimeMillis();
        is = INPUT.isEmpty() ? System.in : new ByteArrayInputStream(INPUT.getBytes());
        out = new PrintWriter(System.out);

        new Main().solve();
        out.flush();
        long G = System.currentTimeMillis();
        tr(G - S + " ms");
    }

    private static boolean eof() {
        if (lenbuf == -1) return true;
        int lptr = ptrbuf;
        while (lptr < lenbuf)
            if (!isSpaceChar(inbuf[lptr++])) return false;

        try {
            is.mark(1000);
            while (true) {
                int b = is.read();
                if (b == -1) {
                    is.reset();
                    return true;
                } else if (!isSpaceChar(b)) {
                    is.reset();
                    return false;
                }
            }
        } catch (IOException e) {
            return true;
        }
    }

    private static byte[] inbuf = new byte[1024];
    static int lenbuf = 0, ptrbuf = 0;

    private static int readByte() {
        if (lenbuf == -1) throw new InputMismatchException();
        if (ptrbuf >= lenbuf) {
            ptrbuf = 0;
            try {
                lenbuf = is.read(inbuf);
            } catch (IOException e) {
                throw new InputMismatchException();
            }
            if (lenbuf <= 0) return -1;
        }
        return inbuf[ptrbuf++];
    }

    private static boolean isSpaceChar(int c) {
        return !(c >= 33 && c <= 126);
    }

    private static int skip() {
        int b;
        while ((b = readByte()) != -1 && isSpaceChar(b))
            ;
        return b;
    }

    private static double nd() {
        return Double.parseDouble(ns());
    }

    private static char nc() {
        return (char) skip();
    }

    private static String ns() {
        int b = skip();
        StringBuilder sb = new StringBuilder();
        while (!(isSpaceChar(b))) { // when nextLine, (isSpaceChar(b) && b != ' ')
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }

    private static char[] ns(int n) {
        char[] buf = new char[n];
        int b = skip(), p = 0;
        while (p < n && !(isSpaceChar(b))) {
            buf[p++] = (char) b;
            b = readByte();
        }
        return n == p ? buf : Arrays.copyOf(buf, p);
    }

    private static char[][] nm(int n, int m) {
        char[][] map = new char[n][];
        for (int i = 0; i < n; i++)
            map[i] = ns(m);
        return map;
    }

    private static int[] na(int n) {
        int[] a = new int[n];
        for (int i = 0; i < n; i++)
            a[i] = ni();
        return a;
    }

    private static int ni() {
        int num = 0, b;
        boolean minus = false;
        while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'))
            ;
        if (b == '-') {
            minus = true;
            b = readByte();
        }

        while (true) {
            if (b >= '0' && b <= '9') {
                num = num * 10 + (b - '0');
            } else {
                return minus ? -num : num;
            }
            b = readByte();
        }
    }

    private static long nl() {
        long num = 0;
        int b;
        boolean minus = false;
        while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'))
            ;
        if (b == '-') {
            minus = true;
            b = readByte();
        }

        while (true) {
            if (b >= '0' && b <= '9') {
                num = num * 10 + (b - '0');
            } else {
                return minus ? -num : num;
            }
            b = readByte();
        }
    }

    private static void tr(Object... o) {
        System.err.println(Arrays.deepToString(o));
    }
}

Measured memory_bytes: 23248