poj2521---lose money

    #include <stdio.h>
    #include <stdlib.h>

    int rever(int n)
    {
            return -n;
    }


    int main()
    {
        int n,m,p,c;
        while(scanf("%d %d %d %d",&n,&m,&p,&c) !=EOF && n && m && p && c)
        {
            printf("%d\n",rever(m-p-n));
        }
        return 0;
    }