#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e18 + 3;
const int maxm = 1e6 + 7;
long long x, y, m;
long long dp[maxm];
long long ans;
void solve() {
  dp[0] = min(x, y);
  dp[1] = max(x, y);
  long long a = dp[0];
  long long b = dp[1];
  if (x <= 0 && y <= 0) {
    if (m <= b) {
      cout << "0" << endl;
      return;
    } else {
      cout << "-1" << endl;
      return;
    }
  }
  if (a < 0 && b > 0) {
    long long num = (-a) / b;
    a += b * num;
    ans += num;
  }
  if (max(b, a) >= m) {
    cout << "0" << endl;
    return;
  }
  for (long long i = 2;; i++) {
    long long c = a + b;
    if (c >= m) {
      cout << i - 1 + ans << endl;
      return;
    }
    a = min(b, c);
    b = max(b, c);
  }
}
int main() {
  cin >> x >> y >> m;
  solve();
  return 0;
}
