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