Evo of prefix expression

//evolution of prefix expression
#include<stdio.h>
#include<conio.h>
#include<MATH.h>
#include<string.h>
char prefix[50],tp[50];
long int stack[50],top= (-1),ans,a,b;
void alert()
{
	printf("\nþûþ notice :==\n \"If you Enter NUMBER then Enter the space after the number is complete\" \n LIKe : \"+3 33\"  ");
}
void push(long int ch)
{
	top++;
	stack[top]=ch;
}
long int pop()
{
	long int ch;
	ch=stack[top];
	top--;
	return ch;
}
int main()
{
	long int i,j,ans,a,b,amt,l;
	char c[20];
	clrscr();
	alert();
	printf("\n ENter the prefix Expression: ");
	gets(prefix);
	strrev(prefix);
	for(i=0;i<strlen(prefix);i++)
	{
		j=0;
		while(prefix[i]>=48 && prefix[i]<=57)
		{
			c[j]=prefix[i];
			i++;
			j++;

		}
		c[j]=NULL;
		strrev(c);

		amt=atoi(c);

		if(amt!=0)
		{
			printf("\nNO ==>  %ld",amt);
			push(amt);
		}

		switch(prefix[i])
		{
			case ' ':
				break;

			case '^':
				a=stack[top];
				top--;
				b=stack[top];
				ans=pow(a,b);
				stack[top]=ans;
				break;

			case '*':
				a=stack[top];
				top--;
				b=stack[top];
				ans=a*b;
				stack[top]=ans;
				break;

			case '/':
				a=stack[top];
				top--;
				b=stack[top];
				ans=a/b;
				stack[top]=ans;
				break;

			case '+':
				a=stack[top];
				top--;
				b=stack[top];
				ans=a+b;
				stack[top]=ans;
				break;

			case '-':
				a=stack[top];
				top--;
				b=stack[top];
				ans=a-b;
				stack[top]=ans;
				break;

		}

	}
	printf("\nEVO of PREFIX expression ==> %ld",stack[top]);
	getch();

	return 0;
}