#include <stdio.h>

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