#include <iostream>
#include <cstdio>
#include <vector>

#define ll long long
#define x first
#define y second
#define point pair<float,float>
#define INF 0x3f3f3f3f

using namespace std;
vector<point> v;


float area(point a, point b){
    return (a.x*b.y - a.y*b.x);
}

float modul(float a){
    return((a<0)?-a:a);
}

int main() {
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);

    unsigned int n;
    cin>>n;
    point left = {INF, INF}, right{-INF, -INF};
    v = vector<point>(n+1);

    float A = 0;
    float X,Y;
    cin>>X>>Y;
    v[0]={X,Y};
    for(int i=1;i<n;i++){
        cin>>X>>Y;
        v[i]={X,Y};
    }
    v[n]=v[0];

    for(int i=0;i<n;i++)
        A+=area(v[i], v[i+1]);

    cout<<modul(A/2);
    return 0;
}