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#include <bits/stdc++.h>
#define N 100005
using namespace std;
int n, i;
double s, x[N], y[N];
double surf(int i, int j)
{
return (x[j] - x[1]) * (y[i] - y[1]) - (x[i] - x[1]) * (y[j] - y[0]);
}
int main()
{
freopen ("aria.in", "r", stdin);
freopen ("aria.out", "w", stdout);
scanf("%i", &n);
for (i = 1; i <= n; i++)
scanf("%lf%lf", &x[i], &y[i]);
for (i = 2; i <= n - 1; i++)
s += surf(i, i + 1);
printf("%lf", fabs(s));
return 0;
}