#include <bits/stdc++.h>

using namespace std;

int main(int argc, char **argv) {
	int K, N, F;
	scanf("%d%d%d\n", &K, &N, &F);

	int remain = K * N;
	for (int i = 0; i < F; i++) {
		int A;
		scanf("%d", &A);
		remain -= A;
		if (remain < 0) {
			remain = -1;
			break;
		}
	}
	printf("%d\n", remain);
	return 0;
}