#include <iostream>
#include <fstream>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int main()
{int n,i;
float s=0,x,y,a,b,c,d;
f>>n;
f>>a>>b;
c=a;
d=b;
for(i=1;i<=n-1;i++)
    {
        f>>x>>y;
        s=s+c*y-d*x;
        c=x;
        d=y;
    }
s=s+(c*b-d*a)/2;
g<<s/2;
    return 0;
}