#include <cstdio>
#include <cmath>
#include <algorithm>

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

    int n;
    double x1, y1, x2, y2, xf, yf, area = 0;
    scanf("%d", &n);
    scanf("%lf %lf", &x1, &y1);
    xf = x1; yf = y1;
    for (int i = 1; i < n; i++) {
        scanf("%lf %lf", &x2, &y2);
        area += x1 * y2 - x2 * y1;
        x1 = x2; y1 = y2;
    }
    area += x1 * yf - xf * y1;
    printf("%.6lf\n", fabs(area) * 0.5);
    return 0;
}