#include <stdio.h>

void main(void){
    long long int a,b,c;
    scanf("%lld %lld %lld",&a,&b,&c);

    a *= 60;
    c *= 3600;
    int count = 1;
    if(a<=b){
        printf("-1");
    }else{
        while(a*count < b*count+c){
            count++;
        }
        printf("%d",count);
    }
}