import sys
input = sys.stdin.readline

def calc(a,b,c,d,e,f):
    c-=a
    d-=b
    e-=a
    f-=b

    return abs(c*f-d*e)


N=int(input())

P=[list(map(int,input().split())) for i in range(N)]

ANS=0

for i in range(1,N-1):
    a,b=P[0]
    c,d=P[i]
    e,f=P[i+1]

    ANS+=calc(a,b,c,d,e,f)

print(ANS)