c语言系列之和

Question

Sum of series using c language

sample input : 12 15 sample output : 54

sample input : 1 100 sample output : 5050

#include <stdio.h>

int main(){
    
    int a, b;
    
    scanf("%d %d", &a, &b);
    printf("%lld",a*(b+1)/2);
    
    return 0;
}

Answer 1

The %lld format specifier is used to print a long long int . The argument you're passing has type int . Mismatching format specifiers triggers undefined behavior , which in this case will likely result in output you don't expect.

To print an int use %d .

printf("%d",a*(b+1)/2);

Answer 2

Your formula: a*(b+1)/2 is wrong

Try:

((b+1)*b - a*(a-1))/2

(assuming that b >= a and a >= 0 )

So the program could be:

#include <stdio.h>

int main(void)
{
    int a, b;
    
    if (scanf("%d %d", &a, &b) != 2) exit(1);
    if (a < 0) exit(1);
    if (b < a) exit(1);
    printf("%d", ((b+1)*b - a*(a-1))/2);
    
    return 0;
}

Note: The formula can be updated to handle negative values but I'll leave that to OP