#include <bits/stdc++.h>
using namespace std;
int const e = 1e5;
int main() {
  long long x, y, m, mv = 0, s, temp = 0;
  cin >> x >> y >> m;
  if (x <= 0 && y <= 0 && m > 0) {
    cout << "-1";
    return 0;
  }
  if ((x < 0 && x < m) && (y < 0 && y < m)) {
    cout << "-1";
    return 0;
  }
  if (max(x, y) < m)
    if (m > 0)
      if (x < 0 && y > 0) {
        temp = (x / y) * -1;
        x = x % y;
      } else if (y < 0 && x > 0) {
        temp = (y / x) * -1;
        y = y % x;
      }
  while (max(x, y) < m) {
    s = min(x, y);
    if (s == x)
      x += y;
    else
      y += x;
    mv++;
  }
  cout << mv + temp;
  return 0;
}
