mod = 1000000007
eps = 10**-9


def main():
    import sys
    input = sys.stdin.readline

    N = int(input())
    ans = 0
    x0, y0 = map(int, input().split())
    x1, y1 = map(int, input().split())
    for _ in range(N-2):
        x, y = map(int, input().split())
        ans += abs((x - x0) * (y1 - y0) - (x1 - x0) * (y - y0))
        x1, y1 = x, y
    print(ans)


if __name__ == '__main__':
    main()