Program for converting Roman letter into Decimal number

Program:
#include<stdio.h>
#include<string.h>
#include<conio.h>
int digitValue(char);
int main()
{
clrscr();
char roman_Number[1000];
int i=0;
long int number =0;
printf("\nEnter roman letter: ");
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("\nEqual 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;
}

Output:

Enter roman letter : VII
Equal decimal value is : 7