#include <iostream>
#include <fstream>
#include <cmath>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
#define NMAX 100005
int N, i;
double sol;
struct punct
{
    double x,y;
}v[NMAX];
const punct ct = {0,0};
double det(punct A,punct B,punct C=ct)
{
    return (A.x-C.x)*(B.y-C.y)-(B.x-C.x)*(A.y-C.y);
}

int main()
{
    f >> N;
    for(i = 0 ; i < N ; ++i)
        f >> v[i].x >> v[i].y;
    v[N] = v[0];
    for(i = 0 ; i < N ; ++i)
        sol += det(v[i], v[i+1]);
    g << fabs(sol/2);
    return 0;
}