#include<cstdio>
using namespace std;
const int MAXN=100005;
int n;
double arie=0.000;
struct pct{double x,y;};
pct p[MAXN];

void read()
{
    FILE *fin=fopen("aria.in","r");
    fscanf(fin,"%d",&n);
    int i;
    for (i=1; i<=n; ++i)
        fscanf(fin,"%lf%lf",&p[i].x,&p[i].y);
    fclose(fin);
}
void solve()
{
    p[n+1]=p[1];
    for (int  i=1; i<=n; ++i)
        arie+=(p[i].x*p[i+1].y-p[i+1].x*p[i].y);
}
void write()
{
    FILE *fout=fopen("aria.out","w");
    fprintf(fout,"%.4lf",arie/2);
    fclose(fout);
}
int main()
{
    read();
    solve();
    write();
    return 0;
}