#include<stdio.h>

typedef long long int int64;

void run(void){
  int a,b,c;
  scanf("%d%d%d",&a,&b,&c);
  int64 d=(int64)60*a-b;
  if(d<=0){
    printf("-1\n");
  } else {
    printf("%lld\n",((int64)3600*c+d-1)/d);
  }
}

int main(void){
  run();
  return 0;
}