#include <bits/stdc++.h>
using namespace std;
const double pi = acos(-1);
const double tau = 2 * pi;
const double epsilon = 1e-9;
long long X, Y, M;
int main(int argc, const char* argv[]) {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cin >> X >> Y >> M;
  if (X >= M || Y >= M)
    cout << "0\n";
  else if (X + Y <= X && X + Y <= Y)
    cout << "-1\n";
  else {
    long long nOperations = 0;
    if (X < Y) X ^= Y ^= X ^= Y;
    if (Y < 0) {
      nOperations += (-Y / X) + ((-Y) % X != 0);
      if (-Y % X != 0)
        Y = X - (-Y) % X;
      else
        Y = 0;
    }
    if (X < Y) X ^= Y ^= X ^= Y;
    while (X < M) {
      Y = X + Y;
      if (X < Y) X ^= Y ^= X ^= Y;
      ++nOperations;
    }
    cout << nOperations << '\n';
  }
  return 0;
}
