#include <bits/stdc++.h>
using namespace std;
struct my{
    float x,y;
}v[100010];
int main(){
	ifstream fin("aria.in");
	ofstream fout("aria.out");
	ios::sync_with_stdio(false);
	fin.tie(0);
    int n;fin>>n;
    for(int i(1);i<=n;++i)
        fin>>v[i].x>>v[i].y;
    v[n+1].x=v[1].x;
    v[n+1].y=v[1].y;
    float rs;
    for(int i(1);i<=n;++i)
        rs+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    fout<<rs/2;
    return 0;
}