#include <bits/stdc++.h>
using namespace std;
inline int fi() {
  register int c = getchar();
  int x = 0;
  int neg = 0;
  for (; ((c < 48 || c > 57) && c != '-'); c = getchar())
    ;
  if (c == '-') {
    neg = 1;
    c = getchar();
  }
  for (; c > 47 && c < 58; c = getchar()) {
    x = (x << 1) + (x << 3) + c - 48;
  }
  if (neg) x = -x;
  return x;
}
int main() {
  long long x, y, z;
  cin >> x >> y >> z;
  if (x > y) swap(x, y);
  if (y <= 0) {
    puts(y >= z ? "0" : "-1");
  } else if (y >= z)
    puts("0");
  else {
    long long ans = 0;
    if (x < 0) {
      long long t = (-x / y);
      x = (t * y + x);
      ans = t;
    }
    while (y < z) {
      long long tmp = x + y;
      x = y;
      y = tmp;
      if (x > y) swap(x, y);
      ans++;
    }
    cout << ans << endl;
  }
  return 0;
}
