#include <bits/stdc++.h>
using namespace std;
int main() {
  long long x, y, m, hasil, temp;
  cin >> x >> y >> m;
  if (x > y) swap(x, y);
  if ((x >= m) || (y >= m)) {
    hasil = 0;
  } else if (y <= 0) {
    hasil = -1;
  } else {
    if (x < 0) {
      temp = -x / y;
      x += temp * y;
      hasil = temp;
    }
    while (x < m && y < m) {
      ++hasil;
      temp = x + y;
      if (x < y)
        x = temp;
      else
        y = temp;
    }
  }
  cout << hasil << endl;
}
