/*以下是我对问题2521 How much did the businessman lose 而用c语言写的代码，judge online提示时间错误，请高手指点*/
main()
{int N,M,P,C,j,i=0,t[10];
for(;;)
{
scanf("%d%d%d%d",&N,&M,&P,&C);
if (N==0&&M==0&&P==0&&C==0) break;
t[i]=N-M+P;
i+=1;
}
j=i;
for(i=0;i<j;i++)
printf ("%d\n",t[i]);
}