#include <bits/stdc++.h>
using namespace std;
long long x, y, m;
long long process1() {
  long long fa[111], fb[111];
  fa[0] = 1;
  fa[1] = 1;
  fb[0] = 0;
  fb[1] = 1;
  int res = 0;
  long long sum = x;
  int idx = 1;
  while (sum < m) {
    res++;
    sum = fa[idx] * x + fb[idx] * y;
    idx++;
    if (idx > 1) {
      fa[idx] = fa[idx - 1] + fa[idx - 2];
      fb[idx] = fb[idx - 1] + fb[idx - 2];
    }
  }
  return res;
}
long long process2() {
  long long k = -y;
  long long tmp = 0;
  tmp = k / x;
  if (k % x != 0) tmp++;
  y += tmp * x;
  if (x < y) swap(x, y);
  return tmp + process1();
}
int main() {
  cin >> x >> y >> m;
  if (x < y) swap(x, y);
  if (x >= m) {
    printf("0\n");
    return 0;
  }
  if (x == 0) {
    if (x >= m)
      printf("0\n");
    else
      printf("-1\n");
    return 0;
  }
  if (x > 0 && y >= 0) {
    cout << process1() << endl;
    return 0;
  }
  if (x > 0 && y < 0) {
    cout << process2() << endl;
    return 0;
  }
  if (x < 0 && y < 0) {
    if (x > m || y > m)
      printf("0\n");
    else
      printf("-1\n");
  }
  return 0;
}
