#include <bits/stdc++.h>
#define pb push_back

using namespace std;

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

void debug_out() { cerr << '\n'; }
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) { cerr << " " << H; debug_out(T...);}
#define dbg(...) cerr << #__VA_ARGS__ << " ->", debug_out(__VA_ARGS__)
#define dbg_v(x, n) do{cerr<<#x"[]: ";for(int _=0;_<n;++_)cerr<<x[_]<<" ";cerr<<'\n';}while(0)
#define dbg_ok cerr<<"OK!\n"

typedef pair<int,int> pii;
typedef long long int ll;
typedef long double ld;

const int DMAX = 1e5+10;

struct nume{
    ld x,y;
};

nume V[DMAX];

int n;
ld ans;

ll det(nume x,nume y,nume z){
    ld ans;
    ans=1LL*x.x*y.y;
    ans+=1LL*y.x*z.y;
    ans+=1LL*z.x*x.y;
    ans-=1LL*y.y*z.x;
    ans-=1LL*z.y*x.x;
    ans-=1LL*x.y*y.x;
    return ans;
}

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(0);

    int t,i,j;

    fin>>n;
    for(i=1;i<=n;i++)
        fin>>V[i].x>>V[i].y;
    for(i=3;i<=n;i++)
        ans+=det(V[1],V[i-1],V[i]);
    fout<<fixed<<setprecision(7)<<ans/2<<'\n';

    return 0;
}