#include <bits/stdc++.h>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
const int N= 100010;
int n,x[N],y[N];
int64_t aria;
int main()
{
    f>>n;
    for(int i=1;i<=n;i++)
        f>>x[i]>>y[i];
    x[0]=x[n];y[0]=y[n];
    for(int i=1;i<=n;i++)
        aria+=1LL*(y[i-1]+y[i])*(x[i-1]-x[i]);

    if(aria<0)
        aria=-aria;

    g<<aria/2;

    if(aria%2)
        g<<".5\n";
    else
        g<<".0\n";

    return 0;
}