Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>

int find_duplicate(int arr[], int n) {
    // Your logic here
    int sumdiff = 0;
    int squarediff = 0;
    for (int i = 0; i < n; i++){
        sumdiff += (arr[i] - i);
        squarediff += (arr[i]*arr[i] - i*i);
    }
    if (sumdiff != 0){
        return (squarediff/sumdiff + sumdiff)/2;
    }
    return -1;
}

int main() {
    int n;
    scanf("%d", &n);

    int arr[100];
    for (int i = 0; i < n; i++) scanf("%d", &arr[i]);

    int result = find_duplicate(arr, n);
    printf("%d", result);
    return 0;
}

Solving Approach