#include <stdio.h>

int main()
{

        int K, N, F;
        int A[1000];

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


        int sum = 0;

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