#include <bits/stdc++.h>
#include <numeric>
#define ll long long
#define rep(i, n) for (int i = 0; i < (int)(n); i++)


using namespace std;

const ll mod = 1000000007;
const ll INF = (ll)1000000007 * 1000000007;


#define DEBUG

#ifdef DEBUG
#define PRINT(A) std::cout << (#A) << ":" << (A) << std::endl;
#else
#define PRINT(A)
#endif

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


int A, B;
int a[10010][10010], b[1000];
int dp[10010][3];
 
int main()
{
    int N, M; cin >> N >> M;
    int A;
    rep(i,M){
        cin >> A;
        N -= A;
    }

    cout << max(N,-1) << endl;

    return 0;
}
