#include <cstdio>
        using namespace std;
        struct punct
        {
            int x, y;
        };
        punct a[100001];
        int arie (int p, int r)
        {
            int s=(a[p].x*a[r].y-a[r].x*a[p].y)/2;
            return s;
        }
        int main()
        {
            int n, i, x, s=0;
            freopen("aria.in","r",stdin);
            freopen("aria.out","w",stdout);
            scanf("%d",&n);
            for (i=1; i<=n; i++) scanf("%d%d",&a[i].x,&a[i].y);
            for (i=1; i<=n; i++)
            {
                if (i!=n) x=arie(i,i+1);
                else x=arie(n,1);
                s+=x;
            }
            printf("%d\n",s);
            return 0;
        }