How to fix " Expected expression before "=" token"? : turns out to have error "expected expression before '=' token"

#include <iostream>
#define e = 2.71828

using namespace std;

    int main()
{
    int n,b,c;

       cout << "put n: ";
       cin >> n;
       b = e-(1/(e*e));
       for (int a=0; a<=n; a++)
        {if (a==0)
       {b=c;
	      c=1-a*c;}}
       cout << "积分值为：" << c << endl;

       return 0;
}