N = int(input())
X = [0] * N
Y = [0] * N
for i in range(N):
    X[i],Y[i] = list(map(int,input().split()))
S = 0
for i in range(1,N-1):
    S += abs((X[i] - X[0]) * (Y[i+1] - Y[0]) - (X[i+1] - X[0]) * (Y[i] - Y[0]))
print(S)