#include <stdio.h>

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