import std.stdio;
import std.string, std.conv, std.array, std.algorithm;
import std.uni;

void main(){
    // Input
    auto line = readln.split.map!(to!int).array;
    int K = line[0], N = line[1], F = line[2];
    auto A = readln.split.map!(to!int).array;

    // Process
    int ans = K * N - sum(A);

    // Output
    writeln(ans >= 0 ? ans : -1);
}