C program to convert Roman number to decimal number

#include<stdio.h>
#include<conio.h>
#include<string.h>
int digitValue(char);
int main()
{
char roman_Number[1000];
int i=0;
long int number =0;
clrscr();
printf("Enter any roman number (Valid digits are I, V, X, L, C, D, M): \n");
scanf("%s",roman_Number);
while(roman_Number[i])
{
if(digitValue(roman_Number[i]) < 0){
printf("Invalid roman digit : %c",roman_Number[i]);
return 0;
}
if((strlen(roman_Number) -i) > 2){
if(digitValue(roman_Number[i]) < digitValue(roman_Number[i+2])){
printf("Invalid roman number");
return 0;
}
}
if(digitValue(roman_Number[i]) >= digitValue(roman_Number[i+1]))
number = number + digitValue(roman_Number[i]);
else
{
number = number + (digitValue(roman_Number[i+1]) - digitValue(roman_Number[i]));
i++;
}
i++;
}
printf("Its decimal value is : %ld",number);
getch();
return 0;
}
int digitValue(char c)
{
int value=0;
switch(c)
{
case 'I': value = 1; break;
case 'V': value = 5; break;
case 'X': value = 10; break;
case 'L': value = 50; break;
case 'C': value = 100; break;
case 'D': value = 500; break;
case 'M': value = 1000; break;
case '\0': value = 0; break;
default: value = -1;
}
return value;
}