#include <iostream>
#include <fstream>
#include <algorithm>

using namespace std;

int n, l;
bool* arr;

inline bool valid(int cur) {
    for(int i = cur, j = cur; i >= 0 && j <= l; --i, ++j)
        if(arr[i] != arr[j])
            return false;
    return true;
}

int main(void) {
    ifstream fin("device.in");
    ofstream fout("device.out");
    fin >> n >> l;
    arr = new bool[l+1]{};
    int pos;
    for(int i = 0; i <= l; ++i) {
        fin >> pos;
        arr[pos] = true;
    }
    int ans = 0;
    for(int i = 1; i < l; ++i)
        if(valid(i))
            ++ans;
    fout << ans;
    delete []arr;
    return 0;
}