Singular only shows well the first time

[quedicesebas]

<input type="range" ng-model="activity.length" min="1" max="10" step="1"> <span translate translate-n="activity.length" translate-plural="{{$count}} hours">1 hour</span>

I have this and activity.length initial value is 1 ->"1 hour" is show. When activity.length increases -> "{{$count}} hours" is shown, but when returns to 1 -> "1 hours is shown"