#include <bits/stdc++.h>
using namespace std;
int main() {
  long long a, b, m, ans = 0;
  scanf("%I64d%I64d%I64d", &a, &b, &m);
  if (a >= m || b >= m)
    printf("0\n");
  else if (a <= 0 && b <= 0)
    printf("-1\n");
  else {
    if (a < 0 || b < 0) {
      if (a > b) swap(a, b);
      ans = -a / b;
      a += b * ans;
    }
    while (a < m && b < m) {
      if (a > b) swap(a, b);
      ans++;
      a += b;
    }
    printf("%I64d\n", ans);
  }
  return 0;
}
