#include <bits/stdc++.h>
int main() {
  long long x, y, m, r = 0, d, aux;
  scanf("%I64d%I64d%I64d", &x, &y, &m);
  if (x > y) {
    aux = x;
    x = y;
    y = aux;
  }
  if (y >= m) {
    printf("0\n");
    return 0;
  }
  if (y <= 0) {
    printf("-1\n");
    return 0;
  }
  while (y < m) {
    d = (y - x) / y + 1;
    r += d;
    if (m - y * d <= x) break;
    x += y * d;
    aux = x;
    x = y;
    y = aux;
  }
  printf("%I64d\n", r);
  return 0;
}
