20分，有大佬知道为啥用函数不对吗

#include<stdio.h>

int isrun(int num) {
	if ((num % 4 == 0 && num % 100 != 0) || (num % 400 == 0)) return 1;
}

int main() {
	int n, m;
	scanf("%d%d", &n, &m);
	int i,sum=0;
	for (i = n+1; i < m; i++) {
		if (isrun(i) == 1) {
			sum += i;
		}
	}
	printf("%d\n", sum);
	return 0;
}
```c