#include <bits/stdc++.h>

using namespace std;

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

const int NMAX = 1e5;

struct ura{
    long long x, y;
}v[NMAX+5];



int main()
{
    
    int n;
    f >> n;
    
    for(int i=1; i<=n; i++)
        f >> v[i].x >> v[i].y;
        
    
    v[n+1].x = v[1].x;
    v[n+1].y = v[1].y;
    
    long long sum = 0;
    
    for(int i=1; i<=n; i++)
        sum += (v[i].x * v[i+1].y - v[i+1].x * v[i].y);
        
    //sum /= 2;
    
    if(sum % 2 == 0)
        g << abs(sum/2);
        
    else g << abs(sum/2) << ".5";

    return 0;
}