[Link] https://www.acmicpc.net/problem/2166
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#include <algorithm>
#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <cstring>
#include <stack>
#include <cmath>
// #include <bits/stdc++.h>
using namespace std;
#define FASTIO ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define command_param int argc, char *argv[]
#define for0(i, n) for(int i = 0; i < n; i++)
#define for1(i, n) for(int i = 1; i < n; i++)
#define fori(s, e) for(int i = s; i < e; i++)
#define ll long long
#define endl "\n"
#define nulls '\0'
#define dkdk " "
const int MAX = 2002;
const int INF = 987654321;
int n;
vector<pair<int,int>> points;
ll calc_size(pair<int,int> v1, pair<int,int> v2) {
return (ll)v1.first * v2.second - (ll)v1.second * v2.first;
}
int main() {
FASTIO;
cin >> n;
for0(i, n) {
int x, y; cin >> x >> y;
points.push_back(make_pair(x, y));
}
double size = 0;
for1(i, n - 1) {
size += calc_size(
{ points[i].first - points[0].first, points[i].second - points[0].second },
{ points[i+1].first - points[0].first, points[i+1].second - points[0].second }
);
}
cout << fixed;
cout.precision(1);
cout << (abs(size)/2);
return 0;
}