SOURCE

const store = new Vuex.Store({
  state: {
    count: 0
  },

  mutations: {
  	increment: state => state.count++,
    decrement: state => state.count--
  }
})

new Vue({
  el: '#app',
  store,
  computed: {
    count () {
	    return store.state.count
    }
  },
  methods: {
    increment () {
      this.$store.commit('increment')
    },
    decrement () {
    	store.commit('decrement')
    }
  }
})

<script src="https://cdn.bootcss.com/vue/2.4.2/vue.js"></script>
<script src="https://cdn.bootcss.com/vuex/2.3.1/vuex.js"></script>

<div id="app">
  <p>{{ count }}</p>
  <p>
    <button @click="increment">+</button>
    <button @click="decrement">-</button>
  </p>
</div>