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