#include <bits/stdc++.h>
using namespace std;
long long x, y, m;
int main() {
  while (cin >> x >> y >> m) {
    if (x >= m || y >= m) {
      cout << 0 << endl;
      continue;
    }
    if ((x <= 0) && (y <= 0)) {
      cout << -1 << endl;
      continue;
    }
    long long cnt = 0;
    long long tmp = min(x, y);
    x = max(x, y);
    y = tmp;
    if (y < 0) {
      long long absy = abs(y);
      cnt = absy / x + 1LL;
      absy %= x;
      y = x - absy;
    }
    while (!(x >= m || y >= m)) {
      long long tmp = min(x, y);
      x = max(x, y);
      y = tmp;
      y += x;
      cnt++;
    }
    cout << cnt << endl;
  }
  return 0;
}
