#include <cstdio>
#include <cmath>
using  namespace std;
const int Max = 100007;
struct yer
{
double  x,y;
}v[Max];
double sum;
int n;
int main()
{
freopen("aria.in", "r", stdin);
freopen("aria.out", "w", stdout);
scanf("%d", &n);
for(int i = 1; i <= n; ++i)
{
scanf("%lf%lf", &v[i].x, &v[i].y);
}
v[n + 1].x = v[1].x;
v[n + 1].y = v[1].y;
for(int i = 1; i <= n; ++i)
{
sum+= v[i].x * v[i + 1].y - v[i + 1].x * v[i].y;
}
printf("%lf", abs(sum) / 2.0);
return 0;
}