#include<stdio.h>
int main()
{
int X,Y,Z,total;
scanf("%d\n",&X);
scanf("%d\n",&Y);
scanf("%d\n",&Z);
scanf("%d",&total);
total=
if(total<0)
{
printf("invalid input");
}
else
{
printf("%d\n",total);
}
return 0;
}