def main():
    import sys
    input = sys.stdin.read().split()
    idx = 0
    N = int(input[idx])
    idx += 1
    S = list(input[idx])
    idx += 1
    T = list(input[idx])
    idx += 1
    
    if S[0] != T[0] or S[-1] != T[-1]:
        print(-1)
        return
    
    delta = [0] * (N)
    for i in range(N):
        if S[i] != T[i]:
            delta[i] = 1
    
    operations = 0
    
    for i in range(N-2):
        j = i + 1
        if delta[j] == 1:
            if S[i] != S[i+2]:
                print(-1)
                return
            # Perform the operation
            S[j] = 'A' if S[j] == 'B' else 'B'
            operations += 1
            delta[j] = 0
    
    for i in range(N):
        if S[i] != T[i]:
            print(-1)
            return
    
    print(operations)

if __name__ == "__main__":
    main()