Program to convert Roman number to Decimal number

#include<stdio.h>
#include<conio.h>
void main()
{
int i,sum=0;
char c[20];
static int p,r;
clrscr();
printf("Enter the Roman number \n");
gets(c);
for(i=0;c[i]!='\0';i++)
{
switch(c[i])
{
case 'M': sum+=1000,p=1000;break;
case 'D': sum+=500,p=500;break;
case 'C': sum+=100,p=100;break;
case 'L': sum+=50,p=50;break;
case 'X': sum+=10,p=10;break;
case 'V': sum+=5,p=5;break;
case  'I': sum+=1,p=1;break;
default:printf("Invalid entry\n");
getch();
exit(0);
}
if(r<p)
sum=sum-2*r;
r=p;
}
printf("%s is %d",c,sum);
getch();
}