#include <bits/stdc++.h>
using namespace std;
const int N = 90;
long long f[N];
int main() {
  long long x, y, m;
  cin >> x >> y >> m;
  if (x > y) swap(x, y);
  if (m <= y) {
    printf("0\n");
    return 0;
  }
  if (y <= 0) {
    printf("-1\n");
    return 0;
  }
  long long res = 0;
  long long k = (-x) / y + 1;
  k = max(0ll, k);
  x = x + k * y;
  f[1] = 1;
  for (int i = 2; i < N; i++) {
    f[i] = f[i - 1] + f[i - 2];
  }
  res += k;
  for (int i = 1; i < N; i++) {
    if (f[i - 1] * x + f[i] * y >= m) {
      break;
    }
    res++;
  }
  cout << res << "\n";
  return 0;
}
