import std;

void main() {
    int n, m; scan(n, m);
    auto a = readints;
    int ans = n - a.sum;
    writeln(max(ans, -1));
}

void scan(T...)(ref T a) {
    string[] ss = readln.split;
    foreach (i, t; T) a[i] = ss[i].to!t;
}
T read(T)() { return readln.chomp.to!T; }
T[] reads(T)() { return readln.split.to!(T[]); }
alias readint = read!int;
alias readints = reads!int;
