#include <bits/stdc++.h>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
typedef long double ld;
const int lim=1e5+4;
#define x first
#define y second
pair<ld,ld> v[lim];
int main()
{
    int n;
    in>>n;
    for(int i=1;i<=n;++i)
        in>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    ld arie=0.;
    for(int i=1;i<=n;++i)
        arie+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    arie=abs(arie/2.);
    out<<setprecision(5)<<arie;
    return 0;
}