C Series Problem

#include <stdio.h>

int main()
{
    int a,n,d,p,sum;
    int indefinite=0;
    while(indefinite==0){
    printf("\nPlease enter the first digit of the series (Type -0 to close the program): ");
    scanf("%d", &a);
    if(a==-0){
        break;
    }
    printf("Please enter the distance of the digit: ");
    scanf("%d", &d);
    printf("Please enter the total number of digits in the series: ");
    scanf("%d", &n);
    sum=(n/2)*(2*a+(n-1)*d);
    p=((2*sum)/n)-a;
    if(n>=6){
        printf("\nThe series is %d + %d + %d.....%d=%d\n", a,a+d,a+2*d,p,sum);
    }
    else if(n==5){
        printf("\nThe series is %d + %d + %d + %d + %d=%d\n", a, a+d, a+2*d, a+3*d, p,sum);
    }
    else if(n==4){
        printf("\nThe series is %d + %d + %d +%d=%d\n", a,a+d,a+2*d,p,sum);
    }
    else if(n==3){
        printf("\nThe series is %d + %d + %d=%d\n", a,a+d,a+2*d,sum);
    }
    else if(n==2){
        printf("\nThe series is %d + %d=%d\n", a,p,sum);
    }
    else{
        printf("\nThe only digit of the series is %d and the sumation is %d\n", a,sum);
    }
    }

    return 0;


}