#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:200000000")
using namespace std;
template <typename T>
inline T Abs(T x) {
  return (x >= 0) ? x : -x;
}
template <typename T>
inline T sqr(T x) {
  return x * x;
}
template <typename T>
inline string toStr(T x) {
  stringstream st;
  st << x;
  string s;
  st >> s;
  return s;
}
template <typename T>
inline int bit(T mask, int b) {
  return (b >= 0 && (mask & (T(1) << b)) != 0) ? 1 : 0;
}
inline int nextInt() {
  int x;
  if (scanf("%d", &x) != 1) throw;
  return x;
}
inline long long nextInt64() {
  long long x;
  if (scanf("%I64d", &x) != 1) throw;
  return x;
}
inline double nextDouble() {
  double x;
  if (scanf("%lf", &x) != 1) throw;
  return x;
}
const int INF = (int)1E9;
const long long INF64 = (long long)1E18;
const long double EPS = 1E-9;
const long double PI = 3.1415926535897932384626433832795;
const int MAXN = 100100;
int main() {
  long long x = nextInt64();
  long long y = nextInt64();
  long long m = nextInt64();
  if (x > y) swap(x, y);
  if (max(x, y) >= m) {
    cout << 0 << endl;
    return 0;
  }
  if (max(x, y) <= 0) {
    cout << -1 << endl;
    return 0;
  }
  long long ans = 0;
  if (x < 0 && y > 0) {
    ans = (min(m, 0LL) - x) / y;
    x += ans * y;
  }
  for (int i = 0; i < (int)(10000); i++) {
    if (max(x, y) >= m) {
      cout << i + ans << endl;
      return 0;
    }
    if (max(x, y) <= 0) break;
    if (x > y) swap(x, y);
    x = x + y;
  }
  cout << -1 << endl;
  return 0;
}
