#include <stdio.h>
#define CEIL(a,b) ((a+b-1)/b)

int main() {
  long long a, b, c, ans;
  scanf("%lld %lld %lld", &a, &b, &c);
  a*=60;
  c*=60*60;

  ans=CEIL(c, (a-b));

  printf("%lld\n", ans>=0 ? ans: -1);
  return 0;
}