#include <stdio.h>

main() {
    int K, N, F;
    int A[100];
    
    scanf("%d %d %d", &K, &N, &F);
    for (int i=0; i<F; i++) {
        scanf("%d", &(A[i]));
    }
    
    int sum = 0;
    for (int i=0; i<F; i++) {
        sum += A[i];
    }
    
    if (sum > K*N)
        printf("-1\n");
    else
        printf("%d\n", K*N-sum);
}