AOJ0079.cpp

#include <bits/stdc++.h>
using namespace std;

using VS = vector<string>;    using LL = long long;
using VI = vector<int>;       using VVI = vector<VI>;
using PII = pair<int, int>;   using PLL = pair<LL, LL>;
using VL = vector<LL>;        using VVL = vector<VL>;

#define ALL(a)  begin((a)),end((a))
#define RALL(a) (a).rbegin(), (a).rend()
#define SZ(a) int((a).size())
#define SORT(c) sort(ALL((c)))
#define RSORT(c) sort(RALL((c)))
#define UNIQ(c) (c).erase(unique(ALL((c))), end((c)))
#define FOR(i, s, e) for (int(i) = (s); (i) < (e); (i)++)
#define FORR(i, s, e) for (int(i) = (s); (i) > (e); (i)--)
//#pragma GCC optimize ("-O3") 
#ifdef YANG33
#include "mydebug.hpp"
#else
#define DD(x) 
#endif
const int INF = 1e9;                          const LL LINF = 1e16;
const LL MOD = 1000000007;                    const double PI = acos(-1.0);

/* -----  2019/07/19  Problem: AOJ 0079 / Link: https://onlinejudge.u-aizu.ac.jp/challenges/search/volumes/0079  ----- */


using Double = long double;
const double EPS = 1e-9;

typedef struct Point {
	Double x, y;
	Point(Double x_, Double y_) : x(x_), y(y_) {}

	bool operator < (const Point& a) const {
		return fabs(x - a.x) < EPS ? y + EPS < a.y : x + EPS < a.x;
	}

	bool operator > (const Point& a) const {
		return a < *this;
	}

	bool operator == (const Point& a) const {
		return !(a < *this) && !(a > *this);
	}

	Point operator + (const Point& a) const {
		return Point(x + a.x, y + a.y);
	}

	Point operator - (const Point& a) const {
		return Point(x - a.x, y - a.y);
	}

	Point operator * (const Point& a) const {
		return Point(x * a.x, y * a.y);
	}

	Point operator / (const Point& a) const {
		return Point(x / a.x, y / a.y);
	}

	Point operator * (const Double& d) const {
		return Point(x * d, y * d);
	}

	Point operator / (const Double& d) const {
		return Point(x / d, y / d);
	}
} Vector;

Double cross(const Vector& a, const Vector& b) {
	return a.x * b.y - a.y * b.x;
}

// 多角形構造体
struct Polygon {
	vector< Point > vert;
	Polygon(int n = 3) : vert(n, Point(0, 0)) {}

	Double area() const {
		Double res = 0;
		int v_size = size();

		FOR(i, 0, v_size) res += cross(at(i), at(i + 1));

		return fabs(res) / 2.;
	}

	Point at(int i) const {
		i %= size();
		return vert[i];
	}

	int size() const { return SZ(vert); }
};

stringstream deletecommma(string line) {
	istringstream ss(line);
	string s;
	stringstream input;
	while (getline(ss, s, ',')) {
		input << s; input << " ";
	}
	return input;
}

int main() {
	Polygon Ps(0);
	string in;
	while (cin >> in) {
		double x, y; auto input = deletecommma(in);
		input >> x >> y;
		Ps.vert.emplace_back(x, y);
	}
	cout << fixed << setprecision(10) << Ps.area() << "\n";
}