#include <bits/stdc++.h>
using namespace std;
int main() {
  long long a, b, m;
  cin >> a >> b >> m;
  if (a > b) swap(a, b);
  if (b >= m) return 0 * puts("0");
  if (b <= 0) return 0 * puts("-1");
  long long ans = 0;
  if (a < 0) {
    ans = abs(a) / b;
    a = a + ans * b;
  }
  long long fib[3];
  if (a > b) swap(a, b);
  fib[0] = a;
  fib[1] = b;
  if (b >= m) return 0 * printf("%lld\n", ans);
  for (int i = 2; i <= 10000; i++) {
    fib[0] = fib[1] + fib[0];
    if (fib[0] > fib[1]) swap(fib[0], fib[1]);
    if (fib[1] >= m) return 0 * printf("%lld\n", ans + i - 1);
  }
  puts("-1");
  return 0;
}
