#include <cstdio>
using namespace std;
int x[100010],y[100010];
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int n,i;
    double aria=0;
    scanf("%d",&n);
    for(i=1;i<=n;++i)
        scanf("%d%d",&x[i],&y[i]);
    for(i=1;i<n;i++)
        aria+=(x[i]*y[i+1]-x[i+1]*y[i]);
    aria+=(x[n]*y[1]-x[1]*y[n]);

    aria/=2;
    printf("%lf",aria);
    return 0;
}