#include<bits/stdc++.h>
// using namespace std;
#if __has_include(<atcoder/all>)
#include<atcoder/all>
// using namespace atcoder;
#endif
#define int long long

#pragma region header

#pragma region alias

using lint = long long;
using ll = long long;
using P = std::pair<int,int>;
template<class T> using prique = std::priority_queue<T,std::vector<T>,std::greater<T>>;

#pragma endregion

#pragma region macros

#define rep(i, n) for(int i = 0;i<(int)(n);i++)
#define REP(i, m, n) for(int i = (m);i<(int)(n);i++)
#define drep(i, n) for(int i = (n)-1;i>=0;i--)
#define DREP(i, m, n) for(int i = (m)-1;i>=(int)(n);i--)
#define all(v) (v).begin(),(v).end()
#define reall(v) (v).rbegin(),(v).rend()

template<class T, class U>
bool chmax(T& a,const U b) {
    if(a < b) {
        a = b;
        return true;
    }
    return false;
}

template<class T, class U>
bool chmin(T& a,const U b) {
    if(a>b) {
        a = b;
        return true;
    }
    return false;
}

std::map<int,int> prime_div(int n) {
    std::map<int,int> mp;
    if(~n&1) while(~n&1) n>>=1,mp[2]++;
    for(int i = 3;i<=std::sqrt(n);i+=2) {
        if(n%i==0) {
            while(n%i==0) {
                n/=i;
                mp[i]++;
            }
        }
    }
    if(n!=1) mp[n]++;
    return mp;
}

bool is_flag(const int &bit, const int &k) { return (bit >> k)&1; }

#pragma endregion

#pragma region constant

constexpr long long inf = 1LL << 61;
constexpr long double eps = 1e-10;
constexpr long double pi = 3.141592653589793238;
constexpr int dx[9] = {1, 0, -1, 0, 1, 1, -1, -1, 0};
constexpr int dy[9] = {0, 1, 0, -1, 1, -1, 1, -1, 0};
constexpr long long mod = 1e9+7;
constexpr long long MOD = 998244353;

#pragma endregion

#pragma region inout

template<class T>
std::ostream& operator<<(std::ostream& stream, const std::vector<T>& v) {
    for(int i = 0; i < (int)(v.size()); i++) {
        stream << v[i];
        if(i != (int)(v.size()) - 1) stream << ' ';
    }
    return stream;
}

template<typename Itr>
inline void print(const Itr& begin, const Itr& end, bool endline = true, const char* BEGIN = "{", const char* mid = ", ", const char* END = "}") {
    if(begin == end) return;
    std::cout << BEGIN << *begin;
    for(Itr itr = begin+1; itr < end; itr++) std::cout << mid << *itr;
    std::cout << END;
    if(endline) std::endl(std::cout);
    return;
}

template<class T>
std::istream& operator>>(std::istream& stream, std::vector<T>& v) {
    for(T& p:v) stream >> p;
    return stream;
}

template<typename Itr>
inline void input(Itr begin, Itr end) {
    for(Itr& itr = begin; itr < end; itr++) std::cin >> *itr;
    return;
}

template<class T, class U>
std::ostream& operator<<(std::ostream& stream, const std::pair<T,U>& pair) {
    return stream << pair.first << ' ' << pair.second;
}

template<class T, class U>
inline void print(const std::pair<T,U>& pair,const bool endline = true, const char* begin = "(", const char* mid = ", ", const char* end = ")") {
    std::cout << begin << pair.first << mid << pair.second << end;
    if(endline) std::endl(std::cout);
    else std::cout << ' ';
    return;
}

template<class T, class U>
std::istream& operator>>(std::istream& stream, std::pair<T,U>& pair) {
    return stream >> pair.first >> pair.second;
}

template<class T, class U>
inline void input(std::pair<T,U>& pair, const bool first = true, const bool second = true) {
    if(first) std::cin >> pair.first;
    if(second) std::cin >> pair.second;
}

#pragma endregion

#pragma region DEBUG

#ifdef _DEBUG
template<class T>
inline void _debug_view(const T& x) noexcept {
    std::cout << x;
    return;
}

template<class T, class U>
inline void _debug_view(const std::pair<T,U>& p) noexcept {
    std::cout << "(";
    _debug_view(p.first);
    std::cout << ", ";
    _debug_view(p.second);
    std::cout << ")";
    return;
}

template<class T>
inline void _debug_view(const std::vector<T>& v) noexcept {
    std::cout << "{";
    for(int i = 0;i<v.size();i++) {
        _debug_view(v[i]);
        std::cout << (i+1 == v.size() ? "" : ", ");
    }
    std::cout << "}";
    return;
}

template<class T, class U>
inline void _debug_view(const std::map<T,U>& mp) noexcept {
    std::cout << "{";
    for(auto itr = mp.begin(); itr != mp.end(); itr++) {
        _debug_view(*itr);
        if(std::next(itr) != mp.end()) std::cout << ", ";
    }
    std::cout << "}";
    return;
}

template<class T> 
inline void _debug_view(const std::set<T>& st) noexcept {
    std::cout << "{";
    for(auto itr = st.begin(); itr != st.end(); itr++) {
        _debug_view(*itr);
        if(std::next(itr) != st.end()) std::cout << ", ";
    }
    std::cout << "}";
    return;
}

#define overload5(_1,_2,_3,_4,_5,name,...) name

#define _debug1(a) {\
    do {\
        std::cout << #a << ": ";\
        _debug_view(a);\
        std::endl(std::cout);\
    }while(0);\
}

#define _debug2(a,b) {\
    do {\
        std::cout << #a << ": ";\
        _debug_view(a);\
        std::cout << ", " << #b << ": ";\
        _debug_view(b);\
        std::endl(std::cout);\
    }while(0);\
}

#define _debug3(a,b,c) {\
    do {\
        std::cout << #a << ": ";\
        _debug_view(a);\
        std::cout << ", " << #b << ": ";\
        _debug_view(b);\
        std::cout << ", " << #c << ": ";\
        _debug_view(c);\
        std::endl(std::cout);\
    }while(0);\
}

#define _debug4(a,b,c,d) {\
    do {\
        std::cout << #a << ": ";\
        _debug_view(a);\
        std::cout << ", " << #b << ": ";\
        _debug_view(b);\
        std::cout << ", " << #c << ": ";\
        _debug_view(c);\
        std::cout << ", " << #d << ": ";\
        _debug_view(d);\
        std::endl(std::cout);\
    }while(0);\
}

#define _debug5(a,b,c,d,e) {\
    do {\
        std::cout << #a << ": ";\
        _debug_view(a);\
        std::cout << ", " << #b << ": ";\
        _debug_view(b);\
        std::cout << ", " << #c << ": ";\
        _debug_view(c);\
        std::cout << ", " << #d << ": ";\
        _debug_view(d);\
        std::cout << ", " << #e << ": ";\
        _debug_view(e);\
        std::endl(std::cout);\
    }while(0);\
}

#define debug(...) overload5(__VA_ARGS__,_debug5,_debug4,_debug3,_debug2,_debug1,)(__VA_ARGS__)

#else
#define debug(...)
#endif

#pragma endregion

#pragma endregion

signed main(void) {
    std::cout << std::fixed << std::setprecision(10);
    int n,m;
    std::cin >> n >> m;
    std::vector<int> v(m);
    std::cin >> v;
    std::vector<long double> dp(m+2,1e18);
    const int p = v.front()-1+m;
    debug(p);
    dp[0] = 0;
    REP(i, 1, m+2) {
        chmin(dp[i],dp[i-1]+((i==m+1?n:v[i-1])-(i>1?v[i-2]:1)));
        const int dist = (i==m+1?n:v[i-1])-(i>1?v[i-2]:1);
        debug(i,dist);
        if(1<i&&i<m+1) {
            debug(i);
            chmin(dp[i],p);
        }
    }
    debug(dp);
    std::cout << dp[m+1] << std::endl;
    return 0;
}