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| #include <iostream>
#include <cmath>
#include <algorithm>
#define ll long long
using namespace std;
long long area(pair<int, int> a, pair<int, int> b, pair<int, int> c) {
return (ll)(b.first - a.first) * (c.second - a.second) - (ll)(c.first - a.first) * (b.second - a.second);
}
int main() {
int n;
pair<int, int> a, b, c;
scanf("%d",&n);
n-=2;
ll ans = 0;
scanf("%d %d %d %d",&a.first,&a.second, &b.first, &b.second);
for(int i = 0; i < n; i++) {
scanf("%d %d",&c.first, &c.second);
ans += area(a,b,c);
b = c;
}
ans = abs(ans);
printf("%lld.%d",ans/2,ans%2 ? 5 : 0);
}
|
