#include <cstdio>

using namespace std;

double v[100010][3];

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int n;
    double s=0;
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
        scanf("%lf%lf",&v[i][1],&v[i][2]);
    v[n+1][1]=v[1][1];
    v[n+1][2]=v[1][2];
    for(int i=1;i<=n;i++)
        s=s+1LL*(0-v[i][1])*(0-v[i+1][2])-1LL*(0-v[i+1][1])*(0-v[i][2]);
    printf("%lf",s/2.0);
    return 0;
}