def  cal(z):
    x=z[0][0]
    y=z[0][1]
    x1=z[1][0]-x
    y1=z[1][1]-y
    x2=z[2][0]-x
    y2=z[2][1]-y
    return abs(y1*x2 - x1*y2)
    
        

n=int(input())
lis=[]
ans=0
for i in range(n):
    t=list(map(int,input().split()))
    lis.append(t)
    if len(lis) < 3:
        continue
    san=[]
    for j in [0,-2,-1]:
         san.append(lis[j])

    ans += cal(san)
print(ans)