C语言结果为负数，溢出

Question

为什么下面的结果sum的值会是负数，用了long int 也还是一样

#include<stdio.h>
int main()
{
   int fbi(int n);
   int sum = 0;
   int n = 0;
    scanf_s("%d", &n);
    sum = fbi(n);
    printf("sum=%d\n", sum);
    return 0;
}
 int fbi(int n)
{
    int a = 1;
    int b = 2;
    int c = 0;
    if (n == 1)
        return c = 1;
    if (n == 2)
        return c = 2;
    while (n > 2)
    {
        c = a + b;
        a = b;
        b = c;
        n--;
    }
    return c;
}

Answer 1

你输入的几