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