#include <bits/stdc++.h>
using namespace std;
#pragma comment(linker, "/STACK:255000000")
bool firstout = 1;
template <class T>
T &minn(T &a, T b) {
  if (b < a) a = b;
  return a;
}
template <class T>
T &maxx(T &a, T b) {
  if (a < b) a = b;
  return a;
}
int &madd(int &a, int b) {
  a += b;
  if (a >= 1000000007) a -= 1000000007;
  return a;
}
int &msub(int &a, int b) {
  a -= b;
  if (a < 0) a += 1000000007;
  return a;
}
int &mmult(int &a, int b) { return a = (long long)a * b % 1000000007; }
int mdiv(long long a, long long b, long long m) {
  a = (a % m + m) % m;
  b = (b % m + m) % m;
  if (a % b == 0) return a / b;
  return (a + m * mdiv(-a, m, b)) / b;
}
long long n, m, q;
int A[1012];
int main() {
  int i, j, k;
  int d;
  long long a, b, c;
  int ts;
  for (ts = 1; scanf("%"
                     "I64d",
                     &(a)) > 0;
       ++ts) {
    scanf(
        "%"
        "I64d",
        &(b));
    scanf(
        "%"
        "I64d",
        &(n));
    if (a > b) swap(a, b);
    if (b >= n)
      printf(
          "%"
          "d",
          (0));
    else if (b <= 0)
      printf(
          "%"
          "d",
          (-1));
    else {
      long long res = 0;
      if (a < 0) res += (-a + b - 1) / b, a += res * b;
      for (; b < n; ++res, c = b + a, a = b, b = c)
        ;
      printf(
          "%"
          "I64d",
          (res));
    }
    printf("\n"), firstout = 1;
  }
  return 0;
}
