#include <stdio.h>
#include <stdlib.h>

struct Point{
    int x, y;
} v[100002];

inline double abs(double a){
    if(a<0)
        return -a;
    return a;
}

int main(){
    FILE*fi,*fo;
    fi=fopen("aria.in","r");
    fo=fopen("aria.out","w");
    int n;
    fscanf(fi,"%d", &n);
    for(int i=1;i<=n;i++)
        fscanf(fi,"%d%d", &v[i].x, &v[i].y);
    v[n+1]=v[1];

    double st=0;
    for(int i=1;i<=n;i++){
        double s=-v[i].x*v[i+1].y+v[i+1].x*v[i].y;
        s/=2;
        st+=s;
    }
    fprintf(fo,"%.5lf", abs(st));
    fclose(fi);
    fclose(fo);
    return 0;
}