#include <stdio.h>
#define nmax 100010
using namespace std;
struct date { float x,y; };
int n,i; float sol=0;
date t[nmax];
inline float abss(float x) { if (x<0) return (-x); else return x; }
int main() {
freopen("aria.in","r",stdin);
freopen("aria.out","w",stdout);
scanf("%d",&n);
for (i=1;i<=n;i++) scanf("%f %f",&t[i].x,&t[i].y);
t[n+1]=t[1];
for (i=1;i<=n;i++)
    sol=sol+(t[i].x*t[i+1].y-t[i].y*t[i+1].x);
sol=abss(sol)/2;
printf("%f",sol);
return 0;
}