#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <inttypes.h>
#include <endian.h>
#include <vector>

typedef unsigned int uint_t;
typedef long double real;

template<typename T> class Adder
{
	std::vector<T> v;

 public:
	void clear() {
		v.clear();
		v.push_back(0);
	}

	Adder() {
		clear();		
	}

	void add(T x) {
		size_t i = 0;
		for (i = 0; v[i]; i++) {
			x += v[i];
			v[i] = 0;
		}
		v[i] = x;
		if (i + 1 == v.size())
			v.push_back(0);
	}

	T value() {
		T res = v[0];
		size_t l = v.size();
		for (size_t i = 1; i < l; i++)
			res += v[i];
		return res;
	}
};


static void work(uint64_t n0, uint64_t n1)
{
	assert (n0 < n1);
	assert ((n1 - n0) % 2 != 0);

	Adder<real> s;
	uint64_t buf[2];

	for (uint64_t i = n0; i <= n1; i += 2) {
		size_t cnt = fread(buf, sizeof(buf[0]), 2, stdin);
		assert (cnt == 2);
		buf[0] = be64toh(buf[0]);
		buf[1] = be64toh(buf[1]);
//		fprintf(stderr, "p0=%" PRIu64 ", p1=%" PRIu64 "\n", buf[0], buf[1]);
		real p0 = buf[0], p1 = buf[1];
		real d = (p0 - (p1 - p0) * i) / (p0 * p1);
		s.add(d);
	}
	printf("%.24Lf\n", s.value());

}




int main(int ac, char **av)
{
	uint64_t as[2] = {0, 0};
	assert (ac == 3);
	for (uint_t i = 0; i < 2; i++) {
		char *eptr = 0;
		as[i] = strtoull(av[1 + i], &eptr, 0);
		if (as[i] == 0 || eptr == av[1 + i] || eptr == 0 || *eptr != 0) {
			fprintf(stderr, "wrong argument %u: '%s' \n", i, av[1 + i]);
		}
		fprintf(stderr, "as[%u] = %" PRIu64 " \n", i, as[i]);
	}

	assert (as[0] < as[1]);
	assert ((as[1] - as[0]) % 2 != 0);

	work(as[0], as[1]);
	


	return 0;
}