C program to extract the substring from a given string

#include <Stdio.h> 
 #include <conio.h> 
 int main() 
 { 
    char str[100], substr[100]; 
    int i=0, j=0, n, m; 
    clrscr (); 
    printf("\n Enter the main string: "); 
    gets (str); 
    printf("\n Enter the position from which to start the substring: "); 
    scanf ("%d", &m) ; 
    printf("\n Enter the length of the substring: ") ; 
    scanf ("%d", &n); 
    i=m; 
    while(str[i] !='\0' &&n>=0) 
    { 
      substr[j] = str[i]; 
      i++; 
      j ++; 
      n--; 
    } 
       substr[j] = '\0'; 
       printf ("\n The substring is : "); 
       puts (str); 
      getch(); 
     return 0; 
 }