import java.util.Scanner;

public class Main {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
int A = sc.nextInt(); //고정 비용
 int B = sc.nextInt(); //가변 비용
 int num = 0; // 판매 대수
 int C = sc.nextInt(); // 노트북 판매 비용
 int sales = C * num; //총수입
 int element = A + (B* num); //총비용

 for (int i = 0 ; i <= num; i++) {
if (sales <= element) {
num++;
} else if ( sales > element ) {
break;
}
}

if ( sales > element ) {
System.out.println(num);
} else {
System.out.println(-1);
}
}
}