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

m = 5*(10**6)
j = np.array([np.arange(-m//2, m//2)]*n)
x_plus = j.T + input_array[:, 0]
sin = np.sin(x_plus) / x_plus
sin_sum = np.sum(sin, axis=0)

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

print(res)