#include <stdio.h>
typedef long long ll;

ll myceil(ll a, ll b){
        if(a%b==0) return a/b;
        else return a/b+1;
}

int main(){
        ll A,B,C;
        scanf("%lld %lld %lld",&A,&B,&C);
        if(60*A == B){
                puts("0");
        }else if(60*A < B){
                printf("-1");
        }else{
                printf("%lld\n",myceil(60*60*C,60*A-B));
        }
}