#include <stdio.h>
#include <stdlib.h>

struct punct {
    long double x, y;
};

int main(void)
{
    FILE* in = fopen("aria.in", "r");
    FILE* out = fopen("aria.out", "w");

    int n;
    long double ans = 0;
    fscanf(in, "%d", &n);

    struct punct puncte[n + 1];

    for (int i = 0; i < n; i++) {
        fscanf(in, "%Lf %Lf", &puncte[i].x, &puncte[i].y);
    }

    puncte[n] = puncte[0];

    for (int i = 0; i < n; i++) {
        ans += (puncte[i].x * puncte[i + 1].y - puncte[i + 1].x * puncte[i].y) / 2.0;
    }

    fprintf(out, "%.6Lf\n", ans);

    return 0;

}