#include <bits/stdc++.h>
#define pii pair<int,int>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

struct puncte{
    int x, y;
}v[100200];

int n,s;

int det(puncte a,puncte b,puncte c){
    int X1=a.x, Y1=a.y;
    int X2=b.x, Y2=b.y;
    int X3=c.x, Y3=c.y;
    return (X2-X1)*(Y3-Y1)+(Y2-Y1)*(X3-X1);
}

int32_t main()
{
    f>>n;
    for(int i=1; i<=n; i++)
        f>>v[i].x>>v[i].y;
    v[0]=v[n];
    for(int i=1; i<n; i++)
        s+=det(v[0],v[i],v[i+1]);
    g<<fixed<<setprecision(5)<<abs(s)/2.0<<'\n';
    return 0;
}