The number of census statistics

Contains the initial population, birth rate, death rate and the number of years

#include <iostream>
using namespace std;
float fum(float m, int n)
{
 float t = 1;
 for (int i = 1; i <= n; i++)
 {
  t *=1 + m;
 }
 return t;
}
float growthRate(float B, float D)
{
 float r;
 r = B - D;
 return r;
}
long int estimatedPopulation(long int p, float a, float b, int n)
{
 if (p < 2)
  return 0;
 else if (a < 0 || b < 0)
  return 0;
 else
  return (int)p * fum(growthRate(a, b), n);
}
void main()
{
 cout << "亲，请您分别输入初始人口,出生率,死亡率和年数N:";
 int a, d;
 float b, c;
 cin >> a >> b >> c >> d;
 if (a < 2||b < 0||c < 0)
  cout << "程序接收负的出生率、负的死亡率和小于2的人口数量,请重新输入";
 else
 cout << "人口普查统计人数为:" << estimatedPopulation(a, b, c, d) << endl;
}