How to write a C program for ATM transaction

#include<stdio.h>

int totalThousand =1000;
int totalFiveFundred =1000;
int totalOneHundred =1000;

int main(){

    unsigned long withdrawAmount;
    unsigned long totalMoney;

    int thousand=0,fiveHundred=0,oneHundred=0;

    printf("Enter the amount in multiple of 100: ");
    scanf("%lu",&withdrawAmount);

    if(withdrawAmount %100 != 0){
         printf("Invalid amount;");
         return 0;
    }

    totalMoney = totalThousand * 1000 + totalFiveFundred* 500 +  totalOneHundred*100;

    if(withdrawAmount > totalMoney){
         printf("Sorry,Insufficient money");
         return 0;
    }

    thousand = withdrawAmount / 1000;
    if(thousand > totalThousand)
         thousand = totalThousand;
    withdrawAmount = withdrawAmount - thousand * 1000;

    if (withdrawAmount > 0){
         fiveHundred = withdrawAmount / 500;
         if(fiveHundred > totalFiveFundred)
             fiveHundred = totalFiveFundred;
         withdrawAmount = withdrawAmount - fiveHundred * 500;
    }

    if (withdrawAmount > 0)
         oneHundred = withdrawAmount / 100;

    printf("Total 1000 note: %d\n",thousand);
    printf("Total  500 note: %d\n",fiveHundred);
    printf("Total  100 note: %d\n",oneHundred);

    return 0;
}

Sample output:
Enter the amount in multiple of 100: 7800
Total 1000 note: 7
Total 500 note: 1
Total 100 note: 3