#include <fstream>
#define M 100010
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
long long x[M],y[M],aria;
int i,n;
int main()
{
    f>>n;
    for(i=1;i<=n;i++)
        f>>x[i]>>y[i];
    x[n+1]=x[1];y[n+1]=y[n];
    for(i=1;i<=n;i++)
        aria+= (x[i]-x[i+1])*(y[i]+y[i+1]);//x[i]*y[i+1]-y[i]*x[i+1];
    if(aria<0)aria *= -1;
    g<<aria/2;
    if(aria%2)
        g<<".50000";
    else
        g<<".00000";
    return 0;
}