#include <fstream>
#include <cmath>
#include <algorithm>
#include <iomanip>
#include <cstdio>
using namespace std;
long double x3[1000001],y3[1000001];
int main()
{
    ifstream f("aria.in");
    ofstream g("aria.out");
    // (baza mare +baza mica)*inaltimea pe 2
    long double s=0;
    int n,i;
   long double minim=1000003;
    f>>n;
    for(i=1;i<=n;i++)
    {
        f>>x3[i]>>y3[i];
        if(minim>y3[i])
            minim=y3[i];
    }
    for(i=1;i<=n;i++)
        y3[i]-=minim;
    for(i=2;i<=n;i++)
        s+=(0.5*(y3[i]+y3[i-1])*(x3[i]-x3[i-1]));
    s+=((x3[1]-x3[n])*(y3[1]+y3[n])/2);
   g<<fixed<<setprecision(6)<<s;
    f.close();
    g.close();
    return 0;
}