#include <bits/stdc++.h>

using namespace std;

int n;
double x2,x1,y3,y2,s,x,y;
int main(){
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d%lf%lf",&n,&x,&y);
    x1=x;
    y3=y;
    for (int i=2;i<=n;i++){
        scanf("%lf%lf",&x2,&y2);
        s+=0.5*(x1*y2-x2*y3);
        x1=x2;
        y3=y2;
    }
    s+=0.5*(x2*y-x*y2);
    cout<<fixed<<setprecision(6)<<s;
    return 0;
}