#include<fstream>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");

struct point{
    int x,y;
}v[100001];

int n,aria;

int main(){
    in>>n;
    for(int i=0;i<n;++i)
        in>>v[i].x>>v[i].y;
    v[n]=v[0];
    for(int i=0;i<n;++i)
        aria+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
    out<<0.5*aria;
    return 0;
}