#include<iostream>
#include<cstdlib>
using namespace std;
int main()
{
      double fch;
      cout<<"vvedite pervuyu tsifru \n";
      cin>>fch;
      fch=abc(fch);
....
}

int a;
a=rand()%10 - 5;

if (a<0) a*=-1; //замена abs