#include <bits/stdc++.h>
int readint() {
  int t = 0;
  char c;
  c = getchar();
  while (c < '0' || c > '9') c = getchar();
  while (c >= '0' && c <= '9') {
    t = (t << 3) + (t << 1) + c - '0';
    c = getchar();
  }
  return t;
}
using namespace std;
int main() {
  int i, j, k, t;
  vector<string> vect;
  long long a, b, m;
  cin >> a >> b >> m;
  if (a >= m || b >= m) {
    puts("0");
    return 0;
  } else if (a <= 0 && b <= 0) {
    puts("-1");
    return 0;
  } else {
    long long count = 0;
    if (b > a) swap(a, b);
    if (b < 0) {
      count = (-b / a) + 1;
      b += count * a;
    }
    while (a < m && b < m) {
      count++;
      (a < b) ? (a = a + b) : (b = a + b);
    }
    cout << count << endl;
  }
  return 0;
}
