#include <stdio.h>
#include <math.h>

#define MAX_N 100000

typedef struct {
    double x, y;
} point;

point v[MAX_N];

int main (void) {
    FILE *f;
    double ans;
    int n;

    f = fopen("aria.in", "r");
    fscanf(f, "%d", &n);
    for (int i = 0; i < n; ++i) {
        fscanf(f, "%lf%lf", &v[i].x, &v[i].y);
    }
    fclose(f);

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

    f = fopen("aria.out", "w");
    fprintf(f, "%.5lf\n", fabs(ans / 2));
    fclose(f);
    return 0;
}