#include <stdio.h>
using namespace std;
int main()
{
    int n;
    scanf("%d", &n);
    int x[110], y[110];
    for (int i = 0; i < n; i++)
    {
        scanf("%d%d", &x[i], &y[i]);
    }
    double ans = 0;
    for (int i = 0; i < n - 1; i++)
    {
        ans += 0.5 * (x[i] * y[i + 1] - x[i + 1] * y[i]);
    }
    printf("%0.0lf", ans);
    return 0;
}