#include <cstdio>

const int nmax=100001;

using namespace std;

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);

    int n,i;
    long double x[nmax],y[nmax];

    scanf("%d",&n);
    for(i=1;i<=n;i++) {
        scanf("%Lf%Lf",&x[i],&y[i]);
    }

    x[n+1]=x[1];
    y[n+1]=y[1];

    long double s=0;
    for(i=1;i<=n;i++) {
        s+=(x[i]*y[i+1]-x[i+1]*y[i])/2;
    }
    if(s<0)
        s=-s;

    printf("%.6Lf\n",s);

    return 0;
}