#include <bits/stdc++.h>

using namespace std;
#define ll long double
ifstream fin("aria.in");
ofstream fout("aria.out");
long long  n,i,a,b;
ll s;
struct arie{
ll x,y;

}v[100001];

int main()
{
    fin>>n;
    if(n<=2) {fout<<0;return 0;}


    for(i=1;i<=n;i++)
    {
    fin>>a>>b;
    v[i].x=a;
    v[i].y=b;

    }

s+=v[n].x*v[1].y-v[1].x*v[n].y;
for(i=1;i<n;i++)
{
s+=v[i].x * v[i+1].y-v[i].y * v[i+1].x;

}

if(s<0) s=-s;
fout<<setprecision(6)<<fixed<<s/2;





    return 0;
}