#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
int main() {
int n, x1, x2, y1, y2, x, y;
double sum=0;
cin >> n >> x >> y;
x1=x;
y1=y;
for (int i=0; i<(n-1); i++) {
cin >> x2 >> y2;
sum=sum+(x1+x2)*(y2-y1);
x1=x2;
y1=y2;
}
sum=sum+(x+x2)*(y-y2);
cout << fixed << setprecision(3) << abs(sum) / 2 << endl;
return 0;
}
I2luY2x1ZGUgPGlvc3RyZWFtPgojaW5jbHVkZSA8aW9tYW5pcD4KI2luY2x1ZGUgPGNtYXRoPgp1c2luZyBuYW1lc3BhY2Ugc3RkOwogCmludCBtYWluKCkgeyAKCWludCBuLCB4MSwgeDIsIHkxLCB5MiwgeCwgeTsgCglkb3VibGUgc3VtPTA7IAoJY2luID4+IG4gPj4geCA+PiB5OyAKCXgxPXg7IAoJeTE9eTsgCglmb3IgKGludCBpPTA7IGk8KG4tMSk7IGkrKykgeyAKCQljaW4gPj4geDIgPj4geTI7IAoJCXN1bT1zdW0rKHgxK3gyKSooeTIteTEpOyAKCQl4MT14MjsgCgkJeTE9eTI7IAoJfSAKCXN1bT1zdW0rKHgreDIpKih5LXkyKTsgCgljb3V0IDw8IGZpeGVkIDw8IHNldHByZWNpc2lvbigzKSA8PCBhYnMoc3VtKSAvIDIgPDwgZW5kbDsKCXJldHVybiAwOyAKfQ==