C++ :

#include <algorithm>
#include <cstdio>

using namespace std;

const int N = 1e5 + 5;

int n;
int a[N];
int last, cnt, mx;

int main() {
    scanf("%d", &n);
    for (int i = 1; i <= n; i++) scanf("%d", &a[i]);
    sort(a + 1, a + n + 1);
    last = a[1];
    cnt = mx = 1;
    for (int i = 2; i <= n; i++) {
        if (a[i] == last) continue;
        if (a[i] == last + 1)
            cnt++;
        else
            cnt = 1;
        last = a[i];
        mx = max(cnt, mx);
    }
    printf("%d\n", mx);
    return 0;
}