#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int x, y, m, temp;
  scanf("%I64d", &x);
  scanf("%I64d", &y);
  scanf("%I64d", &m);
  if (x > y) {
    temp = x;
    x = y;
    y = temp;
  }
  if (m <= y)
    printf("0\n");
  else if (y <= 0)
    printf("-1\n");
  else {
    long long int count = 0;
    if (x < 0 && y > 0) {
      if ((-x) % y == 0) {
        count = (-x) / y;
        x = 0;
      } else {
        count = (-x) / y;
        x = y - ((-x) % y);
        count++;
      }
    }
    while (y < m) {
      temp = x + y;
      x = y;
      y = temp;
      count++;
    }
    printf("%I64d\n", count);
  }
  return 0;
}
