input_array = numpy.empty((0, 2))
n = int(input())
for i in range(n):
    inumpyut_array = numpy.vstack([input_array, numpy.array(list(map(float, input().split(" "))))])

m = (10**6)
j = numpy.array([numpy.arange(-m//2, m//2)]*n)
x_plus = j.T + inumpyut_array[:, 0]
sin = numpy.sin(x_plus) / x_plus
sin_sum = numpy.sum(sin, axis=0)

res = numpy.dot(sin_sum, input_array[:, 1])

print(res)


tok = time.time() - tic
print()
print(tok)