/// Coded by Dhiman Sarker Bappi (Dhimanda) - RMSTU

#include<bits/stdc++.h>
using namespace std;
#define    in_file     freopen("input.txt", "r", stdin)
#define    out_file    freopen("output.txt", "w", stdout)
#define    fast        ios_base :: sync_with_stdio(0), cin.tie(0), cout.tie(0)
#define    F           first
#define    S           second
#define    pb          push_back
#define    popb        pop_back
#define    pf          push_front
#define    popf        pop_front
#define    lcm(a,b)    (a*b)/gcd(a,b)
#define    gcd(a,b)    __gcd(a,b)
#define    pi          2*acos(0)
#define    elif        else if
#define    ll          long long
#define    sp          fixed<<setprecision
#define    nl          '\n'
#define    endl        '\n'
#define    forr(i,b,e) for(int (i)=(b); (i) <= (e); (i)++)
#define    fora(i,n)   for(int (i)=0; (i) < (n); (i)++)
class stdout_cus
{
    short float_prec = 7;
public:
    inline stdout_cus operator<<(const int& arg){ printf("%d",arg); return *this; }
    inline stdout_cus operator<<(const short& arg){ printf("%d",arg); return *this; }
    inline stdout_cus operator<<(const bool& arg){ printf("%d",arg); return *this; }
    inline stdout_cus operator<<(const unsigned int& arg){ printf("%u",arg); return *this; }
    inline stdout_cus operator<<(const unsigned short& arg){ printf("%u",arg); return *this; }
    inline stdout_cus operator<<(const long long& arg){ printf("%lld",arg); return *this; }
    inline stdout_cus operator<<(const unsigned long long& arg){ printf("%llu",arg); return *this; }
    inline stdout_cus operator<<(const double& arg){ printf("%.*f",float_prec,arg); return *this; }
    inline stdout_cus operator<<(const float& arg){ printf("%.*f",float_prec,arg); return *this; }
    inline stdout_cus operator<<(const char& arg){ printf("%c",arg); return *this; }
    inline stdout_cus operator<<(const unsigned char& arg){ printf("%c",arg); return *this; }
    inline stdout_cus operator<<(const char* arg){ printf("%s",arg); return *this; }
    inline stdout_cus operator<<(const std::string& arg){ printf("%s",arg.c_str()); return *this; }
    inline stdout_cus operator<<(const stdout_cus&){ return *this; }
    inline void flush() { fflush(stdout); }
    inline stdout_cus precision(short prec) { float_prec = prec; return *this; }
}cout_cus;
class stdin_cus
{
public:
    inline stdin_cus operator>>(const int& arg){ scanf(" %d",&arg); return *this; }
    inline stdin_cus operator>>(const short& arg){ scanf(" %d",&arg); return *this; }
    inline stdin_cus operator>>(const bool& arg){ scanf(" %d",&arg); return *this; }
    inline stdin_cus operator>>(const unsigned int& arg){ scanf(" %u",&arg); return *this; }
    inline stdin_cus operator>>(const unsigned short& arg){ scanf(" %u",&arg); return *this; }
    inline stdin_cus operator>>(const long long& arg){ scanf(" %lld",&arg); return *this; }
    inline stdin_cus operator>>(const unsigned long long& arg){ scanf(" %llu",&arg); return *this; }
    inline stdin_cus operator>>(const double& arg){ scanf(" %lf",&arg); return *this; }
    inline stdin_cus operator>>(const float& arg){ scanf(" %f",&arg); return *this; }
    inline stdin_cus operator>>(const char& arg){ scanf(" %c",&arg); return *this; }
    inline stdin_cus operator>>(const unsigned char& arg){ scanf(" %c",&arg); return *this; }
    inline stdin_cus operator>>(const char* arg){ scanf(" %s",arg); return *this; }
    inline stdin_cus operator>>( std::string& arg){ char tmp[1000000]; scanf(" %s",tmp); arg = tmp; return *this; }
    inline void flush() { fflush(stdin); }
}cin_cus;

int Int(){int x;scanf("%d", &x);return x;}
#define    cout        cout_cus
#define    cin         cin_cus
#define    Int         Int()
#define    mx          100010

void dhimanda();
int main()
{
    //fast;
    //in_file;
    //out_file;
    int t=1;
    //t=Int;
    forr(i,1,t){
        //printf("Case %d: ", i);
        dhimanda();
    }
    return 0;
}
void dhimanda(){
    ll n,m;
    cin >> n >> m ;
    ll sum=0;
    for(int i=0; i < m ; i++) {
        ll x;
        cin >> x ;
        sum += x;
    }
    if(sum <= n) cout << n-sum <<endl;
    else  cout <<-1<<endl;
}

// Contact : chessdhiman@gmail.com


Measured memory_bytes: 3824