#include<stdio.h>
int main(int argc, char const *argv[]){
	long a,b,c,d,e;
	scanf("%ld %ld %ld",&a,&b,&c);
	d=b-a;
	e=c-b;
	if(e==0){
		printf("%ld\n",b);
	}else{
		printf("%ld\n",e*e/d+c);
	}
	return 0;
}