#include<stdio.h>
#include<stdlib.h>
double area(double(*)[2], int);
main()
{
        int i, n;
        double (*a)[2];
        scanf("%d", &n);
        a = (double(*)[2])malloc(sizeof(double) * 2 * n);
        for(i = 0; i < n; i++){
                scanf("%lf %lf", &a[i][0], &a[i][1]);
        }
        printf("%.1f\n", area(a, n));
        free(a);
}
double area(double (*a)[2], int n)
{
        int i;
        double sum = 0;
        for(i = 0; i < n; i++){
                sum += a[i][0] * a[(i + 1) % n][1] - a[i][1] * a[(i + 1) % n][0];
        }
        return sum / 2;
}