N = int(input())

X = []
Y = []
for _ in range(N):
    x, y = map(int, input().split())
    X.append(x)
    Y.append(y)
    
S = 0
for i in range(N):
    S += X[i] * Y[(i+1)%N] - X[(i+1)%N] * Y[i]
    
print(S)