#include <bits/stdc++.h>
using namespace std;
long long x, y, m, t;
int main() {
  scanf("%lld%lld%lld", &x, &y, &m);
  if (x >= m || y >= m) {
    cout << 0 << endl;
    return 0;
  }
  if (x <= 0 && y <= 0) {
    cout << -1 << endl;
    return 0;
  }
  while (x < m && y < m) {
    if (x > y) {
      swap(x, y);
    }
    if (x + y <= 0) {
      t += abs(x) / y;
      x = x + (abs(x) - abs(x) % y);
    } else {
      x += y;
      t++;
    }
  }
  cout << t << endl;
  return 0;
}
