Display sum of activity duration

For this i have created the script task in business process as following :

var esqManager = UserConnection.GetSchemaManager("EntitySchemaManager") as EntitySchemaManager;

// Create instance of EntitySchemaQuery with root schema "Country".

var esqResult = new EntitySchemaQuery(esqManager, "Activity");

// Define variable for EntitySchemaQuery which will keep subquery for aggregating function result.

EntitySchemaQuery subResultEsq;

// Add subquery column, it will return instance of EntitySchemaQuery for calculation of aggregated function result (count of cities in each country).

esqResult.AddColumn("DurationInMinutes", AggregationTypeStrict.Sum, out subResultEsq);

var update = new Update(UserConnection, "Contact")

        .Set("UsrRating", Column.Parameter(subResultEsq));

        return true;