#include <bits/stdc++.h>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

int n,i;
long double sum;

struct{
    long double x,y;
}v[100005];

int main()
{
fin>>n;
for(i=1;i<=n;i++)
    fin>>v[i].x>>v[i].y;
v[n+1] = v[1];
for(i=1;i<=n;i++)
    sum += (v[i+1].x * v[i].y) - (v[i].x * v[i+1].y);
fout << fixed << setprecision(5) << (double)fabs(sum / 2.0);
    return 0;
}