#include <bits/stdc++.h>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

#define lim 100010
int n;
struct pct {double x,y;} ini[lim];
double rez;

double det (pct A, pct B, pct C)
{
    double d = A.x*B.y + B.x*C.y + A.y*C.x - B.y*C.x - A.x*C.y - A.y*B.x;
    return d;
}

int main()
{
    fin>>n;
    for (int i=1; i<=n; i++)    fin>>ini[i].x>>ini[i].y;
    ini[++n]=ini[1];

    for (int i=1; i<=n; i++)
        rez += (0.5 * det (ini[i], ini[i+1], {0,0}));
    fout<<fixed<<setprecision(5)<<rez;

    return 0;
}