#include <bits/stdc++.h>
using namespace std;
int main() {
  long long x, y, m;
  cin >> x >> y >> m;
  if (x <= 0 && y <= 0) {
    if (y > x) {
      long long tm = x;
      x = y;
      y = tm;
    }
    if (x >= m) {
      cout << "0";
    } else {
      cout << "-1";
    }
    return 0;
  }
  if (x < y) {
    long long tm = x;
    x = y;
    y = tm;
  }
  if (x >= m) {
    cout << "0";
    return 0;
  }
  long long k = 0;
  if (y < 0) {
    if (x >= abs(y)) {
      k = 1;
      y = x + y;
    } else {
      long long yy = abs(y);
      k = (yy / x);
      if (yy % x != 0) k++;
      y = x * k + y;
    }
  }
  long long f1 = 1, f2 = 1;
  long long tp = x;
  while (tp < m) {
    tp = x * f1 + y * f2;
    long long tk = f1;
    f1 = f1 + f2;
    f2 = tk;
    k++;
  }
  cout << k;
  return 0;
}
