#include <stdio.h>
int main()
{
int a=0,b=0,c=0,i=1;
scanf("%d %d %d",&a,&b,&c);
if(b>=c){
printf("-1");
}
else{
for(int i=1;i<=56000000;i++){
if(a+b*i<c*i){
printf("%d",i);
break;
}
}
}
return 0;
}
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