axis = []
for _ in range(int(input())) :
    axis.append(tuple(map(int, input().split())))
axis.append(axis[0])

area = 0
for i in range(len(axis)-1) :
    area += axis[i][0]*axis[i+1][1] - axis[i][1]*axis[i+1][0]

print(round(abs(area)*0.5,1))