/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author Ido Kessler
 */

#include <iostream>

#include <fstream>
#include <vector>
#include <queue>
#include <sstream>
#include <algorithm>
#include <numeric>
#include <functional>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>
#include <bitset>
#include <iomanip>
#include <cstdint>
#include <cassert>

#include <tuple>

using ll = long long int;
using vi = std::vector<int>;
using vvi = std::vector<vi>;
using vll = std::vector<ll>;
using vvl = std::vector<vll>;
using pi = std::pair<int, int>;
using vpi = std::vector<pi>;

#define all(x) (x).begin(), (x).end()
#define fori(i, n) for(int i = 0; i < n; i++)
#define rep(i, s, e) for(int i = (s); i <= (e); ++i)
#define repr(i, s, e) for(int i = (s); i >= (e); --i)
#define pb push_back

template<typename T> void mk_unique(T& l) { if (!std::is_sorted(all(l))) std::sort(all(l)); auto it = std::unique(l.begin(), l.end()); auto d = std::distance(l.begin(), it); l.resize(d); }
template<typename T> T &smax(T& l, const T&r) { if(l<r) l=r; return l; }
template<typename T> T &smin(T& l, const T&r) { if(l>r) l=r; return l; }

constexpr int MOD = 998244353;//1000000007;


namespace io{
class input_reader{
void throw_error(){throw std::runtime_error("No more inputs while reading number");}
template<typename T>
inline void readIntegerValue(T&f){bool positive=true;f=0;char c;
if(!in.get(c))throw_error();
while(c<'0'||c>'9'){if(c=='-')positive=false;if(!in.get(c))throw_error();}
while(c>='0'&&c<='9'){f=(f<<3)+(f<<1)+(c&15);if(!in.get(c))break;}
if(!positive)f*=-1;}
public:
std::istream&in;
explicit input_reader(std::istream&in):in(in){}
inline void read(int&f){readIntegerValue(f);}
inline void read(short&f){readIntegerValue(f);}
inline void read(long int&f){readIntegerValue(f);}
inline void read(long long int&f){readIntegerValue(f);}
inline void read(unsigned int&f){readIntegerValue(f);}
inline void read(unsigned short&f){readIntegerValue(f);}
inline void read(unsigned long int&f){readIntegerValue(f);}
inline void read(unsigned long long int&f){readIntegerValue(f);}
template<typename T>inline void read(std::vector<T>&f){for(auto&i:f)read(i);}
template<typename T,int N>inline void read(std::array<T,N>&res){for(auto&i:res)read(i);}
template<typename T1,typename T2>inline void read(std::pair<T1,T2>&p){in.read(p.first,p.second);}
template<typename T>inline void read(T&f){in>>f;}
template<typename T,typename...Args>inline void read(T&t,Args&...args){read(t),read(args...);}
template<typename T>inline void readArray(int n,T*res){while(n--)read(*(res++));}
template<typename T>inline input_reader&operator>>(T&t){return read(t),*this;}
};
}

namespace io{
class output_writer{
std::ostream&out;
template<typename T>inline void printIntegerHelper(T x){if(x>=10)printIntegerHelper(x/10);out.put(x%10+48);}
template<typename T>inline void printInteger(T x){if(x<0)out.put('-'),x=-x;printIntegerHelper(x);}
template<typename T>inline void printArray(T&ar){bool first=true;for(auto&t:ar)print((first)? first=false,"":" ",t);}
public:
explicit output_writer(std::ostream&out):out(out){}
inline void flush(){out.flush();}
inline void print(){}
inline void print(char x){out.put(x);}
inline void print(short x){printInteger(x);}
inline void print(int x){printInteger(x);}
inline void print(long int x){printInteger(x);}
inline void print(long long int x){printInteger(x);}
inline void print(unsigned short x){printInteger(x);}
inline void print(unsigned int x){printInteger(x);}
inline void print(unsigned long int x){printInteger(x);}
inline void print(unsigned long long int x){printInteger(x);}
template<typename T1>inline void print(const T1&t1){out<<t1;}
template<typename T1>inline void print(const std::vector<T1>&t1){printArray(t1);}
template<typename T1,int N>inline void print(const std::array<T1,N>&t1){printArray(t1);}
template<typename T1,int N,int M>inline void print(const std::array<std::array<T1,M>,N>&t1){for(auto&t:t1)println(t);}
template<typename T1>inline void print(const std::vector<std::vector<T1>>&t1){bool first=true;for(auto&t:t1)print((first)? first=false,"":"\n",t);}
template<typename T,typename...Args>inline void print(const T&t,Args...args){print(t),print(args...);}
template<typename...Args>inline void println(Args...args){print(args...),print('\n');}
template<typename T>output_writer&operator<<(const T&t){return print(t),*this;}
};
}

inline ll pow_mod(ll a,ll b,const ll MOD){ll res=1;for(ll i=1;b;b^=b&i,i<<=1,a=(a*a)%MOD)if(b&i)res=(res*a)%MOD;return res;}
inline ll mod_inv(const ll a,const ll MOD){return pow_mod(a,MOD-2,MOD);}
template<typename T>inline T gcd(T a,T b){T t;while(a!=0)t=a,a=b%a,b=t;return b;}
template<typename T>T lcm(const T &a,const T &b) {assert(a!=0&&b!=0);return a/gcd(a,b)*b;}

namespace math {
    class mod_int{
		int v;
	public:
        mod_int(int v):v(v%MOD){}
        mod_int(unsigned int v):v(v%MOD){}
        mod_int(long long int v):v((int)(v%MOD)){}
        mod_int(unsigned long long int v):v((int)(v%MOD)){}
        mod_int(long int v):v((int)(v%MOD)){}
        mod_int(unsigned long int v):v((int)(v%MOD)){}
        mod_int() = default;
		mod_int(const mod_int& o) = default;
		mod_int(mod_int&& o) = default;
		inline mod_int& operator=(const mod_int& o) = default;
		inline mod_int& operator=(mod_int&& o) = default;

		inline mod_int& operator++() { if(v==MOD-1) v=0; else v++; return *this; }
		inline mod_int& operator--() { if(v==0) v=MOD-1; else v--; return *this; }
		inline mod_int operator++(int) { mod_int t = *this; ++(*this); return t; }
		inline mod_int operator--(int) { mod_int t = *this; --(*this); return t; }

		//inline bool friend operator<(const mod_int&lhs,const mod_int&rhs) { return lhs.v  <  rhs.v; }
		//inline bool friend operator>(const mod_int&lhs,const mod_int&rhs) { return lhs.v  <  rhs.v; }
		//inline bool friend operator<=(const mod_int&lhs,const mod_int&rhs) { return lhs.v <= rhs.v; }
		//inline bool friend operator>=(const mod_int&lhs,const mod_int&rhs) { return lhs.v <= rhs.v; }
		inline bool friend operator==(const mod_int&lhs,const mod_int&rhs) { return lhs.v == rhs.v; }
		inline bool friend operator!=(const mod_int&lhs,const mod_int&rhs) { return lhs.v != rhs.v; }
		//template<typename T> inline bool friend operator<(const mod_int&lhs,const T&rhs) { return  lhs <  mod_int(rhs); }
		//template<typename T> inline bool friend operator>(const mod_int&lhs,const T&rhs) { return  lhs >  mod_int(rhs); }
		//template<typename T> inline bool friend operator<=(const mod_int&lhs,const T&rhs) { return lhs <= mod_int(rhs); }
        //template<typename T> inline bool friend operator>=(const mod_int&lhs,const T&rhs) { return lhs >= mod_int(rhs); }
		template<typename T> inline bool friend operator==(const mod_int&lhs,const T&rhs) { return lhs == mod_int(rhs); }
		template<typename T> inline bool friend operator!=(const mod_int&lhs,const T&rhs) { return lhs != mod_int(rhs); }

		inline mod_int friend operator-(const mod_int&lhs,const mod_int&rhs) { int v=lhs.v-rhs.v; if(v<0) v+=MOD; return {v}; }
		inline mod_int friend operator+(const mod_int&lhs,const mod_int&rhs) { int v=lhs.v+rhs.v; if(v>=MOD) v-=MOD; return {v}; }
		inline mod_int friend operator*(const mod_int&lhs,const mod_int&rhs) { return {(int)((((long long int) lhs.v)*rhs.v)%MOD)}; }
		inline mod_int friend operator/(const mod_int&lhs,const mod_int&rhs) { return {(int)((((long long int) lhs.v)*rhs.inv().v)%MOD)}; }
		inline mod_int friend operator%(const mod_int&lhs,const mod_int&rhs) { return { lhs.v % rhs.v }; }

		template <typename T> inline mod_int friend operator-(const mod_int&lhs,const T&rhs) { return lhs - mod_int(rhs); }
		template <typename T> inline mod_int friend operator+(const mod_int&lhs,const T&rhs) { return lhs + mod_int(rhs); }
		template <typename T> inline mod_int friend operator*(const mod_int&lhs,const T&rhs) { return lhs * mod_int(rhs); }
		template <typename T> inline mod_int friend operator/(const mod_int&lhs,const T&rhs) { return lhs / mod_int(rhs); }
		template <typename T> inline mod_int friend operator%(const mod_int&lhs,const T&rhs) { return {lhs.v % rhs}; }

		template <typename T> inline mod_int& operator-=(const T&rhs) { return *this = *this - rhs; }
		template <typename T> inline mod_int& operator+=(const T&rhs) { return *this = *this + rhs; }
		template <typename T> inline mod_int& operator*=(const T&rhs) { return *this = *this * rhs; }
		template <typename T> inline mod_int& operator/=(const T&rhs) { return *this = *this / rhs; }
		template <typename T> inline mod_int& operator%=(const T&rhs) { return *this = *this % rhs; }

		inline mod_int inv() const { return pow(MOD-2); }
		inline mod_int pow(long long int pw) const { mod_int res = 1, val = v; while(pw) {if(pw&1) res*=val; val*= val; pw>>=1;} return res; }

		inline operator int() const { return v; }

		inline friend std::istream& operator>>(std::istream& in,mod_int&rhs) { in >> rhs.v; rhs.v %= MOD; if(rhs.v<0) rhs.v+=MOD; return in; }
		inline friend std::ostream& operator<<(std::ostream& out,mod_int&rhs) { out << rhs.v; return out; }
	};
}

namespace math {
	mod_int factorial(int n) {
		static std::vector<mod_int> mem;
		static mod_int last;
		if (mem.empty()) last = {1}, mem.push_back(last);
		while (mem.size() <= n) mem.push_back(mem.back() * last++);
		return mem[n];
	}

	mod_int n_choose_k(int n, int k){
		mod_int res =  factorial(n) ;
		mod_int res2 = (factorial(k) * factorial(n-k));
		return res/res2;
	}
}
//#include "points_query/point_query.hpp"
//#include "numeric/numeric.hpp"
//#include "hashes/hashes.hpp"
//#include "graphs/graphs.hpp"
//#include "geometry/geometry.hpp"
//#include "extension_operations/extensions.hpp"
//#include "ranges/ranges.hpp"
using mi = math::mod_int;



using namespace std;

class BHomework {
	class Solver {
	public:
		io::input_reader in;
		io::output_writer out;
		Solver(std::istream &in, std::ostream &out) : in(in), out(out) {}

        void print_case_number() {
            static int t = 0;
//            out <<"Case #" << ++t << ": ";
        }

        void solve() {
            print_case_number();
            // <code here>
            int n,m;
            in>>n>>m;
            vi a(m);
            in>>a;
            int res = 0;
            if(accumulate(all(a),0ll) > n) res = -1;
            else res = n - accumulate(all(a), 0);
            // </code here>
            out.println(res);
        }
	};

public:
	void solve(std::istream &in, std::ostream &out) {
		Solver(in, out).solve();
	}
};

int main() {
	std::ios_base::sync_with_stdio(false), cin.tie(0);
	BHomework solver;
	std::istream& in(std::cin);
	std::ostream& out(std::cout);
	solver.solve(in, out);
	return 0;
}


Measured memory_bytes: 3668