#include <cstdio>

#define bufferSize 1 << 18

char inBuffer[bufferSize];

int index = bufferSize;

__attribute__((always_inline)) char get_char()
{
    if(index == bufferSize)
    {
        fread(inBuffer, 1, bufferSize, stdin);

        index = 0;
    }

    return inBuffer[index++];
}

double get_double()
{
    char c = get_char(), sign = '+'; double integerPart = 0;

    for(; c < 48 || c > 57; c = get_char()) sign = c;

    for(; c > 47 && c < 58; c = get_char()) integerPart = integerPart * 10 + c - '0';

    if(c == '.')
    {
        double fractionalPart = 0, power = 1;

        for(; (c = get_char()) > 47 && c < 58; power *= 10)

            fractionalPart = fractionalPart * 10 + c - '0';

        integerPart += fractionalPart / power;
    }

    return (sign == '-') ? -integerPart : integerPart;
}

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);

    long double N = get_double(), x0 = get_double(), y0 = get_double(), x1, y1, x2, y2, area = 0;

    for(x1 = x0, y1 = y0; --N; x1 = x2, y1 = y2)
    {
        x2 = get_double();

        y2 = get_double();

        area += x1 * y2 - x2 * y1;
    }

    area += x1 * y0 - x0 * y1;

    printf("%.5Lf", area / 2);

    return 0;
}