为什么把int n 改为long long 就AC

#include <iostream>
#include <math.h>
#include<algorithm>
using namespace std;

long long n;
long long sum = 0,a1,a2;
int main() {
    
    
        cin >> a1>>a2>>n;
        long long d = a2 - a1;
        sum = a1 * n + n * (n - 1) * d / 2;
        cout << sum << endl;
  
  
    return 0;
}