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#include <fstream>
#define det(i,j) a[i]*b[j] - b[i]*a[j]
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
const int nmax = 100005;
double a[nmax], b[nmax], sol;
int n, i;
int main() {
f >> n;
for (i = 1; i <= n; i++) {
f >> a[i] >> b[i];
if (i > 1) sol += det(i-1,i);
}
sol += det(n,1);
if (sol<0) sol = -sol;
g << sol/2;
return 0;
}