N=int(input())

P=[]
for _ in range(N):
    x,y=map(int,input().split())
    P.append((x,y))

X,Y=P[0]

V=[]
for x,y in P[1:]:
    V.append((x-X,y-Y))

S=0
for i in range(N-2):
    ux,uy=V[i]
    vx,vy=V[i+1]
    S+=abs(ux*vy-uy*vx)
print(S)