#include <bits/stdc++.h>
using namespace std;
void Protect_Sheep_Solution() {
  int n, m;
  cin >> n >> m;
  char a[n][m];
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < m; j++) {
      cin >> a[i][j];
    }
  }
  int flag = 1;
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < m; j++) {
      if (a[i][j] == 'W') {
        int x[] = {1, -1, 0, 0};
        int y[] = {0, 0, 1, -1};
        for (int k = 0; k < 4; k++) {
          int LR = x[k] + i;
          int UD = y[k] + j;
          if (LR >= 0 && LR < n && UD >= 0 && UD < m) {
            if (a[LR][UD] == 'S') {
              flag = 0;
              break;
            } else if (a[LR][UD] == '.')
              a[LR][UD] = 'D';
          }
        }
      }
      if (!flag) break;
    }
    if (!flag) break;
  }
  if (flag) {
    cout << "YES" << endl;
    for (int i = 0; i < n; i++) {
      for (int j = 0; j < m; j++) cout << a[i][j];
      cout << endl;
    }
    return;
  }
  cout << "NO";
}
void Launch_of_Collider_Solution() {
  int n;
  cin >> n;
  string dir;
  cin >> dir;
  int minn = INT_MAX;
  int x, y;
  cin >> x;
  for (int i = 1; i < n; i++) {
    cin >> y;
    if (dir[i - 1] == 'R' && dir[i] == 'L') {
      int t = (y - x) / 2;
      minn = min(t, minn);
    }
    x = y;
  }
  if (minn == INT_MAX)
    cout << -1;
  else
    cout << minn;
}
void Feed_the_cat_Solution() {
  int hh, mm;
  cin >> hh >> mm;
  int H, D, C, N;
  cin >> H >> D >> C >> N;
  double required = ceil(double(H) / N);
  if (hh >= 20) {
    double cost = required * C * 80.0 / 100;
    cout << cost;
    return;
  }
  double cost1 = required * double(C);
  double time = 0;
  if (mm == 0) {
    time += (20 - hh) * 60;
  } else {
    hh++;
    time += (20 - hh) * 60 + 60 - mm;
  }
  H += D * (time);
  required = ceil(double(H) / N);
  double cost2 = required * double(C) * 80.0 / 100;
  cout << min(cost1, cost2);
}
void Soldier_and_Badges_Solution() {
  int n;
  cin >> n;
  set<int> s;
  int re = 0;
  int a;
  for (int i = 0; i < n; i++) {
    cin >> a;
    while (s.count(a)) {
      a++;
      re++;
    }
    s.insert(a);
  }
  cout << re;
}
void Nicholas_and_Permutation() {
  int n;
  cin >> n;
  int index1, index2;
  int minn = INT_MAX, maxx = INT_MIN;
  int x;
  for (int i = 0; i < n; i++) {
    cin >> x;
    if (x < minn) {
      index1 = i;
      minn = x;
    }
    if (x > maxx) {
      index2 = i;
      maxx = x;
    }
  }
  if (index1 < index2) {
    cout << max(index2, n - 1 - index1);
  } else {
    cout << max(index1, n - 1 - index2);
  }
}
void Save_Energy_Solution() {
  long long k, d, t;
  cin >> k >> d >> t;
  t *= 2;
  d = (k + d - 1) / d * d;
  long long time = 2 * k + (d - k);
  long long timeR = t % time;
  cout.precision(20);
  cout << d * (t / time) +
              (timeR > 2 * k ? (timeR - 2 * k) + k + 0.0 : timeR / 2.0)
       << endl;
}
int main() {
  Save_Energy_Solution();
  return 0;
}
