#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 1000000007;
// constexpr int MOD = 998244353;
constexpr int dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1};
constexpr int dy8[] = {1, 1, 0, -1, -1, -1, 0, 1}, dx8[] = {0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U> inline bool chmax(T &a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U> inline bool chmin(T &a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

std::vector<std::string> split(const std::string &s, const std::string &delimiter) {
  std::vector<std::string> res;
  int pos = 0;
  while (true) {
    int del_pos = s.find(delimiter, pos);
    if (del_pos == s.npos) break;
    res.emplace_back(s.substr(pos, del_pos - pos));
    pos = del_pos + delimiter.length();
  }
  res.emplace_back(s.substr(pos));
  return res;
}

int main() {
  string ubh; cin >> ubh;
  int u = stoi(split(ubh, ",")[0]), b = stoi(split(ubh, ",")[1]), h = stoi(split(ubh, ",")[2]);
  int ans = (u + b) * h;
  cout << ans / 2 << (ans & 1 ? ".50\n" : ".00\n");
  return 0;
}