#include <cstdio>

using namespace std;

double vx[100010],vy[100010],s;
int n,i;

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    scanf("%d",&n);
    for(i=1;i<=n;i++) scanf("%lf%lf",&vx[i],&vy[i]);
    for(i=1;i<n;i++)
        s+=(vx[i]*vy[i+1]-vx[i+1]*vy[i])/2;
    s+=(vx[n]*vy[1]-vx[1]*vy[n])/2;
    if(s<0) s=-s;
    printf("%.5lf",s);
    return 0;
}