N = int(input())
X =[0]*N
Y =[0]*N
S2 = 0
for i in range(N):
    X[i],Y[i]=map(int,input().split())

for i in range(1,N-1):
    S2 += abs(X[0]*Y[i] + X[i]*Y[i+1] + X[i+1]*Y[0] - Y[0]*X[i] - Y[i]*X[i+1] - Y[i+1]*X[0])

print(S2)