def cros(x,y):
    return x[0]*y[1]-x[1]*y[0]
N = int(input())
A = [list(map(int,input().split())) for _ in range(N)]
ans = 0
for i in range(1,N-1):
    x = [A[i][0]-A[0][0],A[i][1]-A[0][1]]
    y = [A[i+1][0]-A[0][0],A[i+1][1]-A[0][1]]
    ans += cros(x,y)
print(ans)